(CNN) -- The battle for Syria's most populous city raged and intensified Sunday, with heavy shelling in Aleppo, according to opposition activists.
The Syrian regime used warplanes in the siege, they said.
Heavy shelling was reported in several neighborhoods in northern and central Aleppo, including Hanano, Shaar, Marjeh and Haidariyeh.
A video posted by one activist showed a large plume of smoke rising from a building close to the citadel, a central building and the city's highest point.
Heavy clashes were under way between rebels and regime forces in neighborhoods near the citadel, opposition activists said.
"The buzzing of the warplanes don't stop," said Bashir Al-Hajji, a spokesman for a Free Syrian Army brigade in Aleppo. "But we already started to move forward from the eastern neighborhoods, and God willing, the fight will be over soon and we will be victorious."
Al-Hajji said clashes raged for hours at various spots near the city center, including close to the presidential palace. He said he is in the Industrial City of Aleppo, about 15 kilometers (nine miles) from the city of Aleppo, which is the commercial capital.
Members of the Free Syrian Army tried to stop approaching troops by attacking them in Idlib province, said Mohamed Said, an Aleppo spokesman for the Syrian Revolution General Commission.
As rebels scrambled to fend off regime forces in Aleppo, more gunfire erupted in other parts of Syria, with at least 124 killed nationwide Sunday, the opposition Local Coordination Committees of Syria said.
The deaths included 59 in Damascus and its suburbs and 25 in Aleppo, the group said.
United Nations Secretary-General Ban Ki-moon said the situation may get worse in Aleppo, describing it as "the epicenter of a vicious battle between the Syrian government and those who wish to replace it."
Rebels in Aleppo exude gritty confidence and possess growing clout. They say they are in charge of significant parts of the city and are working to wrest control of the entire sprawling metropolis from the better-equipped forces of Syrian President Bashar al-Assad.
The regime reported deaths, injuries and the capture of "terrorists" -- who Syria has consistently said are behind attacks -- in several neighborhoods of Aleppo on Saturday.
On Sunday, state-run news agency SANA said Syrian forces were continuing to chase "Gulf and Turkish militias" in Aleppo neighborhoods.
Rebels have launched their own attacks in multiple neighborhoods.
That included a brazen push Friday to seize a state-run broadcasting building. Rebels pushed into the radio and TV complex, and took over parts of it, before eventually withdrawing because of snipers and military shelling, the LCC said.
Around Syria's capital, "armed terrorists" attacked a bus carrying 48 Iranian Shiite pilgrims and kidnapped them, Iranian state media said. Syrian state media also reported the abduction and said it is under investigation.
It is unclear whether the hijacking is linked to the uprising against the Syrian government. The Iranian government is an ally of the al-Assad regime, which has been fighting a rebel movement dominated by Sunnis.
Iran is asking Turkey to step in to help free the pilgrims, according to IRNA, Tehran's state media.
In a phone call to his Turkish counterpart, Foreign Minister Ali Akbar Salehi asked for the "prompt intervention of Ankara" to help release the pilgrims, IRNA said Saturday.
Turkey has tense relations with the Syrian regime and is sympathetic to the rebels. Television network Al Arabiya aired video purporting to show the pilgrims detained by Syrian rebels.
In the video, a commander of the Free Syrian Army says the 48 people abducted are revolutionary guards and not pilgrims. CNN cannot independently verify the authenticity of the video.
Iranian Defense Minister Gen. Ahmad Vahidi insisted that "Iran has no military personnel in Syria. Syria has a very strong army and enjoys the support of the people," according to Iran's semi-official Mehr News Agency.
And Kazem Jalali, a member of the country's national security commission, said Sunday that the abducted pilgrims had traveled to Syria individually and not through official channels, according to Iran's IRNA news agency.
Roughly 17,000 people have been killed since the Syrian conflict first flared in March 2011, when government forces began cracking down on protesters, Ban said last month. Opposition activists put the toll at more than 20,000.
The violence has intensified during the Islamic holy month of Ramadan, which began July 21 in Syria and ends this month, the Red Cross said.
Meanwhile, the Syrian envoy to the United Nations, Bashar al-Jaafari, said his family received numerous death threats in the United States. He has reported the threats to U.S. officials, according to Syrian state media.
CNN's Saad Abedine, Yousuf Basil and Amir Ahmed contributed to this report.