The Syrian army turned its forces on Aleppo today, ordering an armoured column to advance on the country's second biggest city and pounding rebel fighters there with artillery and attack helicopters, opposition activists said.
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.
Two top Syrian diplomats, in the United Arab Emirates and Cyprus, have deserted their posts, becoming the latest officials to abandon the Damascus government, rebels said.
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.
Dr 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 rockets today 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. A mechanised battalion headquartered near Tel started bombarding the town of about 100,000 people before dawn and initial reports indicated residential apartment blocks were being hit, they said.
Opposition sources also reported helicopters and machineguns 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Opposition activists said thousands of troops had withdrawn with their tanks and armoured vehicles from Idlib province near the Turkish border and were headed towards Aleppo.
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.
In Aleppo, helicopters were seen firing missiles throughout yesterday, 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.
Video footage posted by activists appeared to show a warplane firing its guns. Dr 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 16-month-old uprising has entered a new and far more violent phase in the past 10 days since rebels poured into Damascus in large numbers. Last Wednesday, an explosion killed four members of Dr Assad's closest 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 1970 coup.
Western powers have been calling for Dr 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 bitter 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.
Dr Assad's international protector Russia added its voice yesterday 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.
Reuters