Syrian government forces battled on Friday to recapture sections of the Damascus ring road from rebels pressing in on the capital, opposition activists said.
War planes fired rockets around Jobar, Qaboun and Barzeh neighborhoods, the sources said. Heavy fighting was taking place at the Hermalleh junction on the ring road just south of Jobar, which had been seized by the rebels.
Rebel fighters based in the eastern Ghouta region broke through government defensive lines on Wednesday, capturing parts of the road and entering Jobar, 2 km (one mile) from security bases in the heart of the city.
President Bashar al-Assad, struggling to contain a revolt in which 60,000 people have been killed since March 2011, has lost control of large parts of Syria but his forces, backed by air power, have so far kept rebels away from central Damascus.
Captain Islam Alloush of the Liwa al-Islam rebel unit said his fighters did not plan to stay on the road. Their control of surrounding areas already rendered the road useless as an army supply line.
"They are fighting off the regime forces but they do not intend to stay at Hermalleh if their losses mount. The objective of this operation is a slow advance toward Damascus," he told Reuters.
Alloush said the rebels had posted snipers in Jobar, where army roadblocks had been overrun or surrounded.
A university student living in Jobar said control of the Harmalleh junction was changing hands between the rebels and the army. By attacking the road, he said, the rebels had linked Jobar with the eastern Ghouta, a rebel-held expanse of suburbs and farmland adjoining Damascus.
The Local Coordination Committees, an opposition activist group, said 44 people were killed in Damascus on Friday. They said 46 people were killed on Thursday, mostly from army shelling.
With a supply line open to military bases on the coast, Assad's core forces from his minority Alawite sect are still based comfortably in the Qasioun mountains on the northwest edge of Damascus, from where they have been shelling the suburbs.
Rebel commanders say they have made mistakes in the past, entering Damascus and other cities without first cutting army supply lines.
Fawaz Tello, a veteran opposition campaigner connected with rebels in Damascus, said the operation was part of a slow encroachment by rebels on the capital.
"Even if the rebels withdraw from the ring road, it will become, like other parts of the capital, too dangerous for the regime to use it," said Tello, speaking from Berlin.
"We are witnessing a 'two steps forward, one step back' rebel strategy. It is a long way before we can say Assad has become besieged in Damascus, but when another main road is rendered useless for him the noose tightens and his control further erodes."
Assad, aged 47, has been president of Syria since the death in 2000 of his father Hafez al-Assad, the "Lion of Damascus", who had ruled for 30 years.
Resentment against the dynasty's repressive ruled boiled into pro-democracy protests in March 2011 and the country has slid into full-scale civil war, with rebel forces based around the country's Sunni majority. Assad is backed by Shi'te power Iran.
The prospect of a negotiated settlement is slim. But Syrian Information Minister Amra al-Zubi said on Friday the opposition was welcome to come to Damascus to discuss Syria's future in line with Assad's proposals for a national dialogue.
Assad's main foes, who have mostly been driven into exile, had already rejected the proposal.
Zubi did not respond to an initiative by opposition leader Moaz Alkhatib, who heads the Syrian National Coalition, to meet Assad's ceremonial deputy Farouq al-Shara if the authorities began releasing tens of thousands of political prisoners.
Khatib, a moderate cleric from Damascus who met representatives of Assad's main backers, Russia and Iran, in Germany this week, said he will withdraw his initiative on Saturday if the authorities do not release all the women jailed during the revolt.
World powers fear the conflict - the longest and deadliest of the uprisings that spread through the Arab world two years ago - could envelop Syria's neighbors, further destabilizing an the region.