Russian Foreign Minister Sergei Lavrov accused the United States on Wednesday of justifying terrorism against the Syrian government and berated Western nations he said had not condemned attacks that killed top members of Syrian President Bashar Assad's inner circle.

Referring to what he said were comments by US State Department spokeswoman Victoria Nuland indicating such attacks were not surprising given the Syrian government's conduct, Lavrov said, "This is a direct justification of terrorism."

Meanwhile, the Syrian army turned its forces on Aleppo on Wednesday, ordering an armored 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 Syrian President Bashar 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.

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

Syrian forces fired artillery and rockets on Wednesday 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 mechanized 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.

"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 sources also reported helicopters and machine-guns were firing on the neighborhood 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.