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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."
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.
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.
"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.
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.
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.
CNN's Ivan Watson, Salma Abdelaziz, Mohammed Jamjoom, Joe Sterling and Yesim Comert contributed to this report.