The 27-kilometre (17-mile) highway remained perilous a day after troops said they had reopened the key link to the outside world in heavy fighting that followed repeated deadly fire on an a bus carrying airport staff and at least two attacks on UN convoys, a watchdog said.

The fighting sparked mounting expressions of concern from UN officials.

UN chief Ban Ki-moon said the conflict had reached "appalling heights of brutality". UN-Arab League peace envoy Lakhdar Brahimi said Syria was in danger of becoming a "failed state" if a political settlement was not reached soon.

The army shelled both the southwestern outskirts of the capital and the town of Douma in the northeastern suburbs, human rights monitors and opposition activists said.

Douma forms part of the so-called Eastern Ghouta region where troops have gone on the offensive to secure the airport highway.

Analysts say President Bashar al-Assad's regime has been trying to establish a secure perimeter around Damascus at all costs in a bid to be in a position to negotiate a solution to the 20-month conflict.

The repeated firing on the airport road prompted the cancellation of a string of international flights.

Airport officials said flights had resumed on Friday but a military source acknowledged more heavy fighting lay ahead to fully secure the road.