Syrian President Bashar al-Assad's forces used artillery, planes and a helicopter gunship to pound rebel positions in Aleppo on Saturday night, witnesses told Reuters.
The report said that Syrian forces struck at Aleppo's Salaheddine district, a gateway into the city of 2.5 million people that has become the frontline of the conflict that has killed some 18,000 people.
A local rebel commander said his fighters were preparing for a “strong offensive” by government forces on the city.
Meanwhile in the capital Damascus, jets bombarded the city as troops kept up an offensive they began on Friday to storm the last rebel bastion there, a resident said.
Both cities had been relatively free from violence during the 17-month uprising but fighting flared in Damascus after a July 18 bombing which killed four of Assad's inner circle and also erupted in Aleppo.
A rebel commander in Aleppo told Reuters on Saturday he expected a Syrian army attack on rebels “within days.”
“We know they are planning to attack the city using tanks and aircraft, shooting at us for three to four days and they plan to take the city,” Colonel Abdel-Jabbar al-Oqaidi said.
Rebels tried to extend their area of control in Aleppo from Salaheddine to the area around the television and radio station, but were pushed back by Assad's troops, an activist said.
In Damascus, a resident in the Adawi neighbourhood just north of the central Old City reported that jets had pounded an area of the capital on Saturday.
On Friday, the UN General Assembly overwhelmingly voted to condemn the Syrian government and demanded a political transition in Syria.
The 193-nation assembly approved the Saudi-drafted resolution, which expressed “grave concern” at the escalation of violence in Syria, with 133 votes in favor, 12 against and 31 abstentions.
Assad's staunch ally Russia was among the 12 countries that opposed the resolution in the assembly, where no country has a veto but all decisions are non-binding. Others that voted against it included China, Iran, North Korea, Belarus, Cuba and other nations that often criticize the West.
Russia later condemned the resolution on Syria as “harmful,” complaining that it was tantamount to a show of support for rebels fighting to oust Syrian President Bashar al-Assad. UN Ambassador Vitaly Churkin said that the Saudi-drafted resolution “hides blatant support to the armed opposition.”
UN Secretary-General Ban Ki-moon warned on Friday that the reported brutality in Aleppo may amount to crimes against humanity.