Syria sent thousands of troops towards Aleppo on Wednesday, where attack helicopters have been pounding rebel fighters, stepping up its assault on the country's largest city to combat a growing revolt against President Bashar Al Assad.

Syria’s 16-month-old uprising 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.

Assad’s forces have launched massive counter assaults 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.

Syrian forces fired artillery and rocket barrages early on Wednesday at the northern Damascus suburb of Al Tel in an attempt to seize it from rebels, causing mass panic and forcing hundreds of families to flee the area, residents and opposition activists said.

The 216th mechanised battalion headquartered near Tel started bombarding the town of about 100,000 people at 3:15am (0015 GMT) and initial reports indicated residential apartment blocks were being hit, they said.

“Military helicopters are flying now 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 Tel. “Electricity and telephones have been cut off.” Opposition activists said thousands of troops had withdrawn with their tanks and armoured vehicles from Idlib province near the Turkish border and were headed towards Aleppo.

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

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

Turkey said crossings with Syria would be closed from Wednesday in response to worsening security conditions. Fighting around Aleppo has shifted hostilities towards the north and Syrian rebels have seized several posts on their side of the frontier with Turkey in the past week.

In Aleppo, helicopters were seen firing missiles throughout on Tuesday, residents said. Rebels were battling government forces by the gates of the historic old city. Troops fired mortars and shells at rebels armed with rifles and machine guns.

“I heard at least 20 rockets fired, I think from helicopters, and also a lot of machine gun fire,” a resident near one of the areas 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.” Residents said fixed-wing jets had also flown over the city, followed by loud noises, although there were contradictory reports as to whether they had fired. Video footage posted by activists appeared to show a warplane firing its guns.

Assad’s forces have occasionally launched air strikes from fixed-wing jets on other cities during the uprising, but tend to rely on helicopters for air strikes in urban areas.

The uprising has entered a new and more violent phase in the past 10 days since rebels poured into Damascus in large numbers.

Last Wednesday, an explosion killed four members of Assad’s inner circle inside a security headquarters, a blow that wiped out much of the top echelon of his military command structure and shattered the reputation for invulnerability that his family has held since his father seized power in a coup in 1970.

Western powers have been calling for Assad to be removed from power for many months, and now say they believe his days are numbered. But they fear that he will fight to the end, raising the risk of sectarian warfare spreading across one of the world’s most volatile regions.

Syria raised the alarm even further on Monday by confirming that it had chemical and biological weapons. In a statement that may have been intended to reassure the world but seemed to have the opposite effect, it said it would not use poison gas against rebels, only against external threats.

Assad’s international protector Russia added its voice on Tuesday to those of Western countries warning him not to use chemical weapons. Western diplomats said Russia may have pressed Syria to make Monday’s statement after the United States and Israel openly discussed their worries about chemical weapons.

US President Barack Obama said the world would hold Assad and his entourage accountable if they used chemical weapons.

Brigadier General Manaf Tlas, a member of Assad’s inner circle who fled Syria this month, appeared on television in his first public comments since defecting. He called on troops to abandon the government.

“I address you... as one of the Syrian Arab Army’s sons who reject the criminal behaviour of this corrupt regime. The honourable people in the military would not accept these crimes,” he said on Al Arabiya television.

Tlas is a member of the Sunni Muslim majority, and his defection was seen as a sign that the Sunni establishment had abandoned Assad, a member of the Alawite minority sect.

Elsewhere in the country, activists said government troops and pro-Assad militia known as shabbiha had attacked a mosque in a village northwest of the city of Hama.

“Troops and shabbiha left the roadblock on the edge of Shariaa and crossed the main road and began firing automatic rifles on the worshippers as they were entering the mosque,” activist Jamil Al Hamwi said by telephone from the area.

“We have confirmed the names of 15 bodies and it is estimated there is a similar number still to be collected from the streets,” said Hamwi, who uses a pseudonym for security reasons. The account, like others from activists, could not be confirmed. Syria restricts access by international journalists.

At least nine people were killed in army shelling of Al Herak, a town south of Deraa, the cradle of the revolt against more than four decades of Assad family rule, activists said.

Video posted on the Internet showed the shattered bodies of a veiled woman and six children in colourful pyjamas, some of them very young. Four lay on one doctor’s table.

In Damascus, troops were trying to snuff out rebel resistance in several areas, including Barzeh, near the centre, and the southern districts of Hajar Al Aswad, Asali and Qadam.

The ferocity of the Syria conflict has only worsened, with 1,261 people killed since fighting intensified in Damascus on July 15, according to the British-based Syrian Observatory for Human Rights, which compiles reports from anti-Assad activists.

That made last week by far the bloodiest in an uprising in which activists say at least 18,000 people have been killed.