The Syrian army has turned its forces towards Aleppo, ordering an armoured column to advance on the country's embattled second city and pounding rebel fighters with artillery and attack helicopters.

The 16-month revolt against President Bashar al Assad has been transformed from an insurgency in remote provinces into a battle for control of the two main cities, Aleppo and the capital, Damascus, where fighting exploded last week.

Mr Assad's forces have launched massive counter-attacks in both cities. They appear to have beaten rebels back from neighbourhoods in the capital and are turning towards Aleppo, a commercial hub in the north.

One Aleppo resident near an area being shelled, who asked to be identified only by his first name, Omar, said by telephone: "Almost everyone has fled in panic, even my family. I have stayed to try to stop the looters; we hear they often come after an area is shelled."

Syrian forces fired artillery and rockets yesterday at the northern Damascus suburb of al Tel in an attempt to seize it from rebels, causing panic and forcing hundreds of families to flee, residents and opposition activists said.

The 216th mechanised battalion, headquartered near al Tel, started bombarding the area, which has a population of about 100,000, before dawn, and initial reports indicated residential apartment blocks were being hit.

"Military helicopters are flying over the town. People were awakened by the sound of explosions and are running away," Rafe Alam, one of the activists, said by phone from a hill overlooking al Tel. "Electricity and telephones have been cut off."

Opposition sources also reported helicopters and machine guns were firing on the neighbourhood of Hajar al Aswad. The slum lies on the southern outskirts of the capital and has been a haven for rebels sneaking into Damascus from the suburbs.

Opposition activists said thousands of troops had withdrawn with their tanks and armoured vehicles from Idlib province near the Turkish border and were heading towards Aleppo.

Rebels attacked the rear of the troops withdrawing from the north, activist Abdelrahman Bakran said from the area.

Military experts believe an overstretched Syrian army is pulling back to concentrate on fighting insurgents in Aleppo and Damascus, important power centres for the Government, while leaving outlying areas in the hands of rebels.

As hostilities intensified near the Turkish border, Turkey said it was closing its crossing posts, although the United Nations said refugees fleeing Syria would be allowed through.

Russia has accused the US of trying to justify terrorism against the Syrian Government and berated Western nations it claims failed to condemn a bomb attack that killed senior security officials.

Foreign Minister Sergei Lavrov suggested Washington was using the threat of further attacks to push the UN Security Council to place international mediator Kofi Annan's peace plan under Chapter 7 of the UN Charter.

Chapter 7 allows the Security Council to authorise actions ranging from diplomatic and economic sanctions to military intervention, although US officials have said they would prefer the former course of action.