Syrian President Bashar Assad's forces used artillery, planes and a helicopter gunship to pound rebel positions in Syria's biggest city, witnesses said, in a battle that could determine the outcome of the 17-month uprising.

After UN Security Council paralysis on Syria forced peace envoy Kofi Annan to resign last week, and with his ceasefire plan a distant memory, rebels were battered on Saturday by the onslaught they had expected in Aleppo and 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 struck at Aleppo's Salaheddine district, a gateway into the city of 2.5 million people that has become the frontline of an increasingly sectarian conflict that has killed some 18,000 people and could spill into neighboring 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 kept up an offensive they began on Friday to storm the last rebel bastion there, a resident said.

Both cities - vital prizes in the battle for Syria - 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.

On Saturday, 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.

TV station offensive repelled

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.

Syrian television said a large number of "terrorists" were killed and wounded after they tried to storm the broadcaster.

After Annan's resignation, the UN General Assembly voted on Friday to condemn the Syrian government and criticize 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 Assad's foreign enemies were arming the rebels and worsening the violence that has elements of a proxy war between Sunni and Shi'ite Islam which could spill beyond Syrian borders.

Assad is a member of the Alawite faith, an offshoot of Shi'ite Islam that has dominated Syrian politics through more than 40 years of his family's rule in a country that has a Sunni Muslim majority.

The mostly Sunni Muslim Gulf Arab states and Turkey have called for Assad to go. Assad still has the backing of Shi'ite Iran and Lebanon's armed Shi'ite Hezbollah movement.

In Damascus, a resident in the Adawi neighborhood just north of the central Old City reported that jets had pounded an area of the capital on Saturday.

Syrian television said an armed terrorist group had committed a massacre in the Damascus suburb of Yalda. The television station said 20 people had been killed. It was not immediately possible to verify the incident as Syria restricts foreign media access.

A bus-load of 48 Iranian pilgrims were abducted by gunmen in Syria on Saturday, Iranian and Syrian media reported, the latest in a string of kidnappings of visitors from the Islamic Republic.