President Bashar al-Assad's forces used artillery, planes and a helicopter gunship to pound rebel positions in Syria's biggest city today.
It is seen as a battle that could determine the outcome of the 17-month uprising.
After UN Security Council inaction on Syria forced peace envoy Kofi Annan to resign this week, rebels were battered by the onslaught they had expected in Aleppo and in the capital Damascus.
"There is one helicopter and we're hearing two explosions every minute," said a Reuters witness in Aleppo, Syria's commercial hub.
Syrian forces attacked Aleppo's Salaheddine district, a gateway into the city of 2.5m people that has become the frontline of an increasingly sectarian conflict that has killed 18,000 people and could spill into neighbouring countries.
A local rebel commander said his fighters were preparing for a "strong offensive" by government forces on the city.
In Damascus, jets bombarded the capital as troops continued an offensive they began yesterday to storm the last rebel stronghold there, a resident said.
Both cities, crucial in the battle for Syria, had been relatively free from violence during the 17-month uprising, but fighting flared in Damascus after the 18 July bombing which killed four of President Assad's inner circle and also erupted in Aleppo.
"We saw two fighter bombers that are each capable of carrying one bomb fly over the area of Salaheddine and then we heard two explosions," said a witness.
Early this morning, a rebel commander in Aleppo said he expected a Syrian army attack on rebels "within days", echoing the head of the UN peacekeeping department, who said there had been a "considerable build-up of military means".
"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.
"The Syrian army sent snipers and surrounded the TV station and as soon as morning came, the army started shooting. One of our fighters was martyred and four were wounded," said a 19-year-old fighter, who gave his name as Mu'awiya al-Halabi.
Syrian television said a large number of "terrorists" were killed and wounded after they tried to storm the broadcaster.
After Mr Annan's resignation, the UN General Assembly voted yesterday to condemn the Syrian government and criticise the UN Security Council's failure to agree tougher action, in a resolution that Western diplomats said highlighted the isolation of Assad supporters Russia and China.
Russia called the vote a "facade of humanitarian rhetoric" behind which Mr Assad's foreign enemies were arming the rebels and worsening the violence.
Meanwhile, a 48 Iranian pilgrims were abducted by gunmen in Syria today, Iranian and Syrian media reported, the latest in a string of kidnappings of visitors from the Islamic Republic.