Artillery and tanks were reportedly being used to strike rebel positions in the city, which Voice of America called a critical objective for both sides due to its large population and commercial importance.
The Syrian Observatory for Human Rights in Britain said Sunday the fighting was intense in the southwestern reaches of Aleppo. Social media reports from journalists gathered by The Guardian described casualties on both sides and burning military vehicles.
The opposition leadership warned the battle for Aleppo would end in a massacre if the troops loyal to President Bashar Assad prevailed, VOA said. The Syrian National Council Sunday called for an emergency session of the U.N. Security Council on the situation.
Assad's government was also looking to the outside for support. Foreign Minister Walid al-Moallem made an unannounced trip to Iran Sunday where he told reporters the government would indeed take Aleppo.
Iran's foreign minister Sunday warned of "severe consequences" for Syrians and foreign countries allegedly behind Syria's bloody unrest.
After meeting with Moallem in Tehran, Minister Ali Akbar Salehi told a news conference Iran stood by Assad, CNN reported.
Salehi warned that increased instability would roil the entire Middle East should Assad fall. He said expectations of a smooth regime change were "an illusion."
"We call upon the people of the region to be fully aware and not to move in the wrong direction because there will be severe consequences that will go beyond the borders of the region to the outside world," the Iranian minister said.
Syria has maintained the rebellion was being led by Islamic extremists rather than pro-democracy groups.