Air strikes and clashes tested a fragile ceasefire in Syria on Friday as civilians waited for aid and the UN Security Council was to discuss whether to endorse the US-Russian truce.
The accord has been billed as the "last chance" to end the five-year war but it has been marred by a lack of aid deliveries, sporadic violence and friction between Moscow and Washington.
Russia, a key backer of Syrian President Bashar al-Assad, said it was ready to extend the truce set to expire later Friday by 72 hours, despite accusing the United States and rebels of not fulfilling the deal.
"We are prepared to extend the cessation of hostilities for a further 72 hours," senior Russian officer Viktor Poznikhir said.
UN Security Council members were to meet at 2130 GMT for closed-door consultations, diplomats said, after Russia's envoy to the United Nations said Moscow wanted a UN resolution to endorse the deal.
On Friday, two children were among three civilians killed in air strikes on the rebel-held town of Khan Sheikhun in the northwest province of Idlib, the Syrian Observatory for Human Rights said.
Khan Sheikhun, like most of the surrounding province, is controlled by an alliance of rebels, hardline Islamists and jihadists such as the Fateh al-Sham Front, formerly Al-Qaeda's Syrian affiliate.
Under the truce deal, which took effect late Monday, fighting is to halt across the country except where jihadists are present.
Experts say the deal will be particularly difficult to implement in areas where Fateh al-Sham has formed strong alliances with local rebels.
'Window of opportunity' 
Earlier Friday, a barrage of rocket fire and shelling could be heard coming from the rebel-held east Damascus district of Jobar, an AFP correspondent said.
Both the Islamist faction Faylaq al-Sham and Fateh al-Sham are thought to be present there.
"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 told AFP.
State television called the incident a violation of the ceasefire.
The United Nations has described the truce as a "critical window of opportunity" to deliver aid to rebel-held eastern districts of Aleppo city, where around 250,000 civilians are under siege.
The UN had hoped that 40 trucks of food -- enough to feed 80,000 people for one month -- could be delivered to east Aleppo as soon as possible.
But Friday morning, the trucks were still waiting at the border with Turkey, said David Swanson, a spokesman for the UN Office for the Coordination 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 told AFP.
Under the truce deal, the main route for humanitarian assistance into divided Aleppo, the Castello Road, would be demilitarised and aid convoys would enter from Turkey.
A military source said on Friday Syria's army "has carried out its pledge and handed over a number of points to the Russian monitoring teams", but that rebel groups had not withdrawn from their positions.
'World is watching' 
"As humanitarians this is immensely frustrating. We're here, we're on the ground and we're ready to move... The world is watching," Swanson said.
An AFP correspondent said that no movement could be seen on Friday on the rubble-strewn Castello Road. Russian and Syrian government flags were visible in the distance.
The US-Russian 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.
Russia said on Friday that only Moscow and the Syrian regime were fulfilling a truce deal.
"Although the ceasefire agreement is bilateral, only one side is truly implementing it," defence ministry spokesman Igor Konashenkov said in a statement.
The United States accused Syria of blocking aid and warned it will not boost military cooperation with Russia unless Damascus honours the truce agreement.
"Right now, the trucks that could bring them life-saving assistance are idling on the wrong side of the border," President Barack Obama's spokesman Josh Earnest said.
"And that's the direct responsibility of the Assad regime and their benefactors in Moscow."
Obama was due to gather top national security aides later on Friday with the shaky ceasefire set to dominate a meeting ostensibly about countering the Islamic State group.