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 today by 72 hours, despite accusing the United States and rebels of not fulfilling the deal.
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.
Today, 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.
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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.
Experts say the deal will be particularly difficult to implement in areas where Fateh al-Sham has formed strong alliances with local rebels.
Earlier today, 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.
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.