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Northern Syria (CNN) -- Syria's most populous city remained engulfed by fighting Tuesday as opposition groups reported incremental rebel gains and the United Nations said civilian displacement rose.
Fighters attacked Syrian police stations in the central neighborhoods of Salhin and Bab al-Nayrab, and seized control of the buildings after hours of clashes, the opposition Syrian Observatory for Human Rights reported.
At least 40 police officers died in the violence, the observatory said. Deama, an activist who asked that her full name not be used for her protection, said Salhin has been the center of many "aggressive operations by the police and Shabiha militia."
A opposition video, purportedly from the Bab al-Nayrab neighborhood, showed bloody corpses amid rubble and rebels chanting "Allahu akbar," an Arabic phrase for "God is great."
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Amid spurts of gunfire, emboldened and elated fighters also shouted "Hafez Assad, the dog of the Arab Nation" and "the Free Syrian Army forever, stepping on Assad's head."
Hafez Assad, the late leader of Syria, is the father of Syrian President Bashar al-Assad. The Free Syrian Army is an anti-government fighting force.
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One rebel who identified himself as Faris said Shabiha militia and rebels were fighting in Bab al-Nayrab.
Among those fighting for the regime, he said, are people from the well-known al-Berri clan, who have members in parliament. A lot of FSA fighters died in the clashes, Faris said.
Deama said rebels remain in control of the neighborhood of Salaheddine and battled regime forces there Tuesday. The area is in the southwestern part of the city.
Even though clashes engulfed Salaheddine, helicopter shelling had not been reported Tuesday, prompting speculation from opposition people that the regime might be planning a big push in the area, Deama said.
Free Syrian Army deputy commander Malek Kurdi, now in rural Aleppo, said the regime has been trying to storm Salaheddine, but has been regularly repelled by the FSA. He said he's seen the regime forces use rockets, 130 mm shells and 120 mm mortars.
"The situation is good now," Kurdi told CNN in a phone call. "I think the regime's troops are too scared to go in. Eight tanks and BMPs have been destroyed by our fighters in the first attempt by the regime to overrun the neighborhood. The regime forces tried to go in today as well, but they couldn't."
"BMP" is the Russian and Arabic acronym for armored personnel carrier.
Also, he said, FSA fighters overran a big security checkpoint in the town of Anadan just north of the city two days ago and the rebels are working to take control of Minakh military airport in Aleppo.
"If that is achieved successfully, then we can say the entire northern and northeastern part of Aleppo will be liberated," he said.
Kurdi said he believes the rebels are geared up for government strikes from three directions.
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Rebel fighters load an anti-aircraft machine gun on an armored vehicle in Atareb, east of Syria's second-largest city, Aleppo, on Tuesday, July 31. Unrest spread across other volatile regions of the country as forces of President Bashar al-Assad's regime shelled targets and launched raids in and around Damascus, Homs, Daraa and Deir Ezzor.
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"The regime is bringing infantry reinforcements by helicopters to the civilian airport in Aleppo and to Ramousa in Aleppo," he said. "But we are ready, and the FSA fighters are already making advances."
Elsewhere in Aleppo, helicopters fired rockets at several neighborhoods, including Maisar, Bab Road, Ard Hamra, Sakhour and Karm Almuyassar, opposition sources said.
The Syrian Observatory said a sniper killed the leader of a rebel battalion in the Marjeh neighborhood, and the opposition Local Coordination Committees of Syria said the Free Syrian Army and regime soldiers were battling in the Meredian neighborhood.
State-run TV said Syrian forces clashed with "armed terrorist groups" on the outskirts of Aleppo and destroyed nine armored vehicles "with all terrorists inside."
"Our armed forces continue to pursue terrorists in the Salaheddine neighborhood in Aleppo. The operations resulted in inflicting heavy losses among the terrorists and the confiscation of their weapons," state TV said.
Aleppo is the commercial and cultural hub of Syria, and the fight to seize control of the urban center is a major battle in what world powers now regard as a civil war.
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The fighting comes a day after rebels scored a notable victory when they captured an army outpost near Aleppo, taking possession of tanks and crates of ammunition in the process.
Unrest spread across other volatile regions of the country Tuesday, as regime forces shelled targets and launched raids in and around Damascus, Homs, Daraa and Deir Ezzor.
The LCC said at least 49 people have been killed in these regions of Syria on Tuesday. At least 20 of them died in Aleppo. It is not clear if any of them were among those killed in the siege of the police stations.
The regime is trying to wrest territory away from rebels who have been able to establish growing enclaves in northern Syria and control much of the main western highway from Aleppo to the Turkish border.
An estimated 200,000 people in and around Aleppo fled their homes over the weekend, the U.N. refugee agency said Tuesday, citing the International Committee of the Red Cross and the Syrian Arab Red Crescent.
"With armed violence raging in Syria's most populous city, thousands of frightened residents are seeking shelter in schools, mosques and public buildings," the agency said in a briefing.
People calling the U.N. High Commissioner for Refugees office in Damascus report "a lack of safety," "fear of ongoing shelling" and a "lack of access to food, water and sanitation, the agency said.
The United Nations says it has registered more than 129,000 Syrian refugees in Turkey, Jordan, Lebanon, Iraq, Algeria, Morocco and Egypt. The refugees include "growing numbers" from Aleppo who "are fleeing across the Hatay border" into Turkey.
The U.N. refugee agency says thousands of people that it hasn't registered have entered those nations.
For example, the Jordanian government said that about 150,000 refugees have entered Jordan since March of last year, when the conflict began. But the United Nations said more than 38,000 are getting protection and assistance, and the rest aren't registered.
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Only 70 Syrians have approached the U.N. refugee office for help in Algeria, but there are reports of 10,000 to 25,000 Syrians in Algeria, the refugee agency said Tuesday.
Many Iraqi refugees who took refuge in Syria because of violence in their homeland are returning. More than 20,000 people have returned home in the past 10 days.
The unrest in Syria started when al-Assad's security forces launched a violent crackdown on peaceful protests in March 2011. That clampdown spurred a nationwide uprising and led to the appearance of armed rebels, such as groups of military defectors and other fighters battling under the rubric of the Free Syrian Army.
The conflict has claimed almost 17,000 lives, U.N. Secretary-General Ban Ki-moon said last week. Opposition activists put the toll at more than 20,000.
The violence has been decried across the globe, and it is taking its toll in the inner circles of government. A recent bombing at a government building in Damascus killed four top Syrian officials, and several diplomats have abandoned the government.
The top diplomat at the Syrian Embassy in London resigned his post, the British Foreign and Commonwealth Office said Monday. Charge d'Affaires Khaled al-Ayoubi told British officials that he was "no longer willing to represent a regime that has committed such violent and oppressive acts against its own people," the office said in a statement.
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CNN's Ivan Watson, Salma Abdelaziz, Mohammed Jamjoom, Joe Sterling and Yesim Comert contributed to this report.
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Syrian boys run near a building hit by bullets and fire in Atareb.
A member of the Free Syrian Army fires at forces loyal to President Bashar Al-Assad in a district of Aleppo called Salah Edinne on Tuesday.
A member of the Free Syrian Army carries an injured civilian to safety in Aleppo's district of Salah Edinne on Tuesday.
Members of the Free Syrian Army learn that a tank belonging to forces loyal to President Bashar Al-Assad is heading to the area.
A Syrian boy carries bags of bread as people wait outside a bakery near Syria's second-largest city, Aleppo.
A photo released by Syrian Arab News Agency depicts damaged buildings in Homs on Monday, July 30.
A Free Syrian Army fighter takes position Sunday, July 29, in Aleppo as people flee shelling. Intense clashes have been under way for more than a week between the regime and rebels in Aleppo, Syria's commercial and cultural center.
Parts of Syria's largest city saw the fiercest clashes yet in the country's 16-month crisis on Saturday, July 28. About 200,000 people have fled fighting in Aleppo and surrounding areas in the past two days, a U.N. official says.
Fighting leaves vehicles damaged Saturday in the southwestern city of Daraa.
Syrians carry the body of a man allegedly killed in the bombardment of Sukari, southwest of Aleppo, by Syrian regime forces on July 27.
Destruction appears widespread in Homs on Friday, July 27, in a handout photo from the Syrian opposition Shaam News Network.
A Syrian opposition fighter takes aim during clashes with forces loyal to President Bashar al-Assad in Aleppo on Wednesday, July 25.
Family and friends mourn over the body of Usame Mircan, who they say was killed by a Syrian government sniper in Aleppo on Wednesday.
Usame Mircan's mother grieves after he was killed during fighting in Aleppo.
The bodies of men killed during clashes between Syrian rebel fighters and goverment forces lie on the Aleppo street on Thursday, July 26.
Fighters from the Syrian opposition rest at a former primary school in Aleppo on Wednesday.
Residents take cover as fighters from the Syrian opposition clash with forces loyal to President Bashar al-Assad in Aleppo on Wednesday.
Syrian rebels guard a checkpoint in Aleppo on Wednesday.
A damaged portrait of President Bashar al-Assad sits among piles of debris at a checkpoint manned by Syrian rebels in Aleppo on Wednesday.
Syrian rebels drive through Selehattin near Aleppo during clashes with government forces on Monday, July 23.
A Syrian rebel runs through the streets of Selehattin during an attack on a municipal building. The rebel Free Syrian Army says it is attempting to "liberate" several districts of Aleppo.
Syrian rebels work to find snipers during clashes Monday between the opposition and government forces in Selehattin.
Syrian rebels make their way down a street Monday in Selehattin near Aleppo. If they gain control of Aleppo, it would mark a pivotal point in the Syrian crisis.
Syrian rebels take cover behind sandbags during fighting Monday at the entrance to the city of Selehattin.
On Sunday, July 22, a Syrian refugee looks out from a bus as he arrives at a refugee camp in Turkey opposite the Syrian commercial crossing point Bab al-Hawa.
Syrian refugees flee from a refugee camp nicknamed "Container City" on the Turkish-Syrian border in Kilis province, southern Turkey, on Sunday.
Smoke from artillery shelling rises above Jbatha Al-khashab.
An armed Syrian rebel wearing the jersey of FC Barcelona rests with comrades near the northern city of Aleppo on Sunday. The rebel Free Syrian Army announced the start of the battle to "liberate" Aleppo, Syria's commercial hub and a traditional bastion of President Bashar al-Assad's regime.
In this photo released by the Shaam News Network, a truck burns after shelling in the Erbeen suburb of Damascus on Saturday, July 21.
Refugees fleeing the violence in Syria arrive by bus in Baghdad, Iraq, on Saturday.
Turkish soldiers stand guard at the Cilvegozu border gate in Reyhanly that connects to Syria's Bab al-Hawa post. An estimated 120,000 people have fled Syria to Turkey, Iraq, Lebanon and Jordan.
Burned-out trucks at the Bab al-Hawa Syrian border post with Turkey on Friday, July 20. Syrian rebels seized control of the post after a fierce battle with Syrian troops, an AFP photographer at the scene reported.
Syrian soldiers celebrate in the al-Midan area in Damascus on Friday. Syrian regime forces routed rebel fighters from the Damascus neighbourhood of Midan, Syrian state television reported, saying troops had "cleaned" the district of "terrorists."
Journalists are shown a dead body on a government tour of the al-Midan area in Damascus on Friday.
Members of Syria security forces rest in the al-Midan area in Damascus on Friday.
Syrian army soldiers hang their national flag in a partially destroyed neighborhood in the al-Midan area in Damascus.
Smoke hangs in the air in a partially destroyed neighborhood in the al-Midan area in Damascus.
Members of Syria security forces pose for photographers in the al-Midan area in Damascus after driving out the rebel fighters.
Syrian residents take goods from a truck that rebels captured at the Bab al-Hawa border crossing with Turkey on Friday.
A picture released by the official Syrian Arab News Agency on July 19 shows Syrian General Fahd al-Freij meeting with President Bashar al-Assad in Damascus after his swearing-in ceremony as defense minister.
A man holds up a picture of President Bashar al-Assad at a former police station in Atareb after clashes between Syrian soldiers and Free Syrain Army near Aleppo on Thursday, July 19. Rebels seized control of border crossings with Iraq on Thursday, dealing a new blow to al-Assad, as China and Russia dismayed the West by blocking U.N. action against his regime.
People walk along the street in Atareb amidst damage caused by clashed between Syrian soldiers and the Free Syrian Army.
A Syrian man checks the former police station of Syrian regime after a clash at Atareb on Thursday.
Smoke ascends from from alleged shelling of the Syrian village of Jebata al-Khashab as seen from the hill village of Buqaata in the Israeli-annexed Golan Heights on Thursday.
The death toll in Syria on July 12 reached 287, making it the bloodiest day in Syria since the uprising began. As it has done consistently, Syrian state television blamed "armed terrorist groups" for the killings.
A Syrian woman sits with her grandson outside a damaged building after attacks in the Syrian village of Treimsa on July 13, 2012. More than 200 people were massacred in the town, according to activists.
A Syrian demonstrator holds an opposition flag during a protest in Damascus on July 2, 2012. There have been increasing reports of violence in the Syrian capital.
Syria's President Bashar al-Assad waves as he arrives for a speech to Syria's parliament in Damascus on June 3, 2012. The embattled president denied that government forces were behind the "outrageous" massacre in Houla.
People gather at a mass burial on May 26, 2012 for victims reportedly killed during an artillery barrage from Syrian forces in Houla. The attack left at least 108 people dead, including nearly 50 children, according to the United Nations.
Members of the Free Syrian Army return to Qusayr on May 12, 2012 after an attack on Syrian regime forces in the village of Nizareer, near the Lebanese border in Homs.
A U.N. observer speaks with Syrian rebels and civilians in the village of Azzara on May 4, 2012, days before the country's parlianemtary polls were held against a backdrop of unrest.
Thousands of Syrians wave their national flag and hold portraits of President Bashar al-Assad and Lebanon's Hezbollah chief Hassan Nasrallah, right, during a rally to show support for their leader on March 29, 2012 in Damascus.
Syrian rebel fighters man a checkpoint leading into the town of Taftanaz in the rebel stronghold province of Idlib on March 20, 2012.
A Free Syrian Army rebel mounts his steed in the Al-Shatouria village near the Turkish border in northwestern Syria on March 16, 2012, a year after the uprising began. The Free Syrian Army is an armed opposition group made up largely of military defectors.
Syrian refugees walk across a field before crossing into Turkey on March 14, 2012. International mediator Kofi Annan called for an immediate halt to the killing of civilians in Syria as he arrived in Turkey for talks on the crisis.
A day after the twin suicide bombings, Syrian mourners pray over the coffins of the 44 people killed during a mass funeral in Damascus.
A Syrian man who was wounded in a suicide attack rests at a hospital in Damascus on December 23, 2011. Suicide bombers hit two security service bases in the Syrian capital, killing dozens of people.
Arab foreign ministers attend an emergency meeting at the Arab League headquarters in Cairo on October 16, 2011, to discuss the crisis in Syria.
U.S. Secretary of State Hillary Clinton speaks to the media in Washington on August 18, 2011. Clinton said U.S. sanctions on Syrian oil "strike at the heart of the Syrian regime."
Syrian youths wave national flags while army troops drive out of Daraa on May 5, 2011. During a week-long military lockdown of the town, dozens of people were reportedly killed in what activists described as "indiscriminate" shelling on the city.
Syrians in Damascus protest in the street on March 25, 2011, after clashes with government forces in Daraa left several dead.
Supporters of Syrian President Bashar al-Assad rally on April 1 in Istanbul, Turkey, as delegates from dozens of countries gather to push for ways to end the deadly violence in Syria. The United Nations estimates more than 10,000 people have been killed since the beginning of the crisis in March 2011. The conflict is now being labeled a civil war by the International Committee of the Red Cross.
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