Fierce fighting and clashes between regime forces and rebels has been reported on the eastern edge of the Syrian capital, despite a fragile truce across the country, AFP reported a military source said.
"The Syrian army is blocking an attack by armed groups that tried to enter the capital's east via Jobar ... leading to intense clashes and rocket fire," a military source said.
A barrage of rocket fire and shelling could be heard coming from the Jobar district, a rebel-held eastern suburb of Damascus.
The district has been a battleground for more than two years and nearly all of its pre-war population has fled.
The Syrian Observatory for Human Rights monitoring group also reported the clashes and said more than 21 shells and rockets hit parts of Jobar.
Two shells also hit the Bab al-Sharqi neighbourhood of Damascus but did not result in any casualties, the Observatory said.
Rami Abdel Rahman, who heads the Britain-based monitor group, said both Islamist faction Faylaq al-Sham and the Fateh al-Sham Front - formerly al-Qaeda's Syrian affiliate - were present in Jobar.
He did not have immediate information on casualties.
Observers have noted that the deal will be particularly difficult to implement in areas where Fateh al-Sham has formed strong alliances with local rebels.
Earlier it was reported that food aid for desperate civilians in eastern Aleppo remained stuck on the Syrian border, on the fourth day of the ceasefire in the war-ravaged country.
Reports said no movement could be seen on the rubble-strewn main route for humanitarian assistance in to divided Aleppo.
The UN had hoped that 40 trucks of food - enough to feed 80,000 people for one month - could be delivered to besieged rebel-held eastern parts of the city as soon as possible.
This morning, the trucks were still waiting at the border with Turkey, said David Swanson, a spokesman for the UN Office for the Co-ordination of Humanitarian Affairs.
"The challenge we continue to face, and this is the very sad reality, is ensuring all parties to the conflict, and those with influence over them, are in agreement," he said.
An estimated 250,000 people still live in east Aleppo.
Under the US-Russia truce deal, the main road leading into those neighbourhoods would be demilitarised and aid convoys would enter from Turkey.
Russia yesterday said government forces had begun to withdraw from the area and accused rebel fighters of failing to pull back as agreed.
"As humanitarians this is immensely frustrating. We're here, we're on the ground and we're ready to move... The world is watching," Mr Swanson said.
The ceasefire deal calls for the truce to be renewed every 48 hours, and for Washington and Moscow to begin unprecedented joint targeting of jihadists if it lasts a week.
The cessation of hostilities appears to be largely holding across the country, though Syria's mainstream opposition has yet to formally sign on.
More than 300,000 people have been killed since Syria's conflict erupted in March 2011.