Air strikes and clashes tested a fragile ceasefire in Syria into Saturday as civilians waited for aid and tensions mounted between the deal's brokers Russia and the United States.
In New York, the UN Security Council cancelled an urgent meeting that had been called to discuss whether to endorse the truce, billed as the "last chance" to end the five-year war that has killed 300,000 people.
The closed-door consultations were scrapped after Moscow and Washington failed to agree over disclosing details of the ceasefire to the council.
US Secretary of State John Kerry had earlier called his Russian counterpart Sergei Lavrov and condemned "repeated and unacceptable delays of humanitarian aid," spokesman John Kirby said.
Kerry told Lavrov that Washington "expects Russia to use its influence on the (President Bashar al-) Assad regime to allow UN humanitarian convoys to reach Aleppo and other areas in need.
"The secretary made clear that the United States will not establish the Joint Implementation Centre with Russia unless and until the agreed terms for humanitarian access are met," Kirby said.
President Barack Obama also voiced "deep concern" that the Syrian regime continued to block humanitarian aid.
The ceasefire has been marred by a lack of aid deliveries and sporadic violence, including three civilians killed Friday.
If the truce, which began Monday, lasts seven days and humanitarian access is granted, Russia and the US are to work together to target militants including the Islamic State group (IS) and former Al-Qaeda affiliate Fateh al-Sham Front.
Under the truce deal, Moscow must put pressure on Assad and Washington must work with Syrian rebels to silence their guns.
Earlier Friday, Russia said that only Moscow and the Syrian regime were fulfilling the deal.
"Although the ceasefire agreement is bilateral, only one side is truly implementing it," defence ministry spokesman Igor Konashenkov said.
However, Russia said it was still ready to extend the truce set to expire late Friday by 72 hours.
France and other council members have said they must first learn more details about the deal before considering whether to endorse it.
The Pentagon also said Friday that dozens of US Special Operations Forces have been deployed to Syria's border with Turkey to fight IS, at Ankara's request, in support of Turkey's army and "vetted" Syrian rebels.
Two children were among three civilians killed in air strikes Friday on the rebel-held town of Khan Sheikhun in Idlib province, 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 Fateh al-Sham.
Under the truce, fighting is to halt across the country except where militants are present.
But experts say the deal will be particularly difficult to implement in areas where Fateh al-Sham has formed strong alliances with local rebels.
Earlier in the day, 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 Observatory said three Islamist fighters and four members of the regime forces were killed.
The UN has called the truce 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 there as soon as possible.
But on Friday, 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.
Under the truce deal, the main route into divided Aleppo, the Castello Road, would be demilitarised and aid convoys would enter from Turkey.
A military source said 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.
"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.