As aid remains held up on the border and ceasefire violations mount, the UN warns "the world is watching".
Air raids and fighting tested a fragile ceasefire in Syria into Saturday as civilians waited for aid and tensions mounted between Russia and the United States, who brokered the truce.
In New York, the UN Security Council cancelled an urgent meeting on Friday that had been called to discuss whether to endorse the deal, billed as the "last chance" to end a five-year war that has killed hundreds of thousands of people and forced millions from the country.
The closed-door consultations were scrapped after Moscow and Washington failed to agree on how disclose 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," State Department spokesman John Kirby said.
Kerry told Lavrov that Washington expected Russia to use its influence on the government of President Bashar al-Assad to press it to allow UN aid convoys to reach Aleppo and other areas, according to Kirby.
"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, referrring to a deal to set up a joint committee to enable the US and Russia to coordinate attacks on the Islamic State of Iraq and the Levant (ISIL) and Jabhat Fateh al-Sham armed groups.
US President Barack Obama also voiced "deep concern" that the government continued to block humanitarian aid.
The ceasefire has been marred by a lack of aid deliveries and sporadic violence, in which three civilians were killed on Friday.
If the truce, which began on Monday , lasts seven days and aid access is granted, Russia and the US say they will work together to target ISIL, also known as ISIS, and Jabhat Fateh al-Sham, the group formerly known as al-Nusra Front.
Under the deal, Moscow must put pressure on Assad and Washington must work with the Syrian rebels it supports to silence their guns.
Earlier on Friday, Russia said that only Moscow and the Syrian government were fulfilling the deal.
"Although the ceasefire agreement is bilateral, only one side is truly implementing it," defence ministry spokesman Igor Konashenkov said.
Russia said, though, that it was ready to extend the truce, which is due to expire late on Friday, by 72 hours.
Russia accused the US "of not doing their part to get rebel forces to pull back from Castello Road, that vital link that aid trucks will eventually travel along into besieged Aleppo," Al Jazeera’s Charles Stratford, reporting from Gaziantep along the Turkey-Syria border, said.
“Also, the Russian defence ministry describing an incident last night saying that Syrian forces close to that road came under attack by the rebels using small arms fire, and… were forced to redeploy to initial positions to protect any kind of demilitarised zone that was beginning to happen there."
Members of the UN Security Council have said they need more details about the deal before deciding whether to endorse it.
The Pentagon also said on Friday that dozens of US Special Operations Forces have been deployed to Syria’s border with Turkey to fight ISIL, at Ankara’s request, in support of Turkey’s army and "vetted" Syrian rebels.
Three civilians, including two children, were killed in air raids on Friday on the rebel-held town of Khan Sheikhun in Idlib province, according to the UK-based Syrian Observatory for Human Rights (SOHR).
Under the truce deal, fighting is to stop across the country except where ISIL and Jabhat Fatah al-Sham are present.
Experts say the deal will be particularly difficult to implement in areas where Jabhat Fateh al-Sham has formed strong alliances with other rebel groups.
’The world is watching’
Hours-long fighting and shelling erupted in neighborhoods on the edges of Damascus on Friday, with activists and residents calling the clashes the heaviest in the Syrian capital in weeks.
Fighting between government troops and rebels was concentrated in the neighbourhood of Jobar, next to Qaboun, where rebels, including Jabhat Fatah al-Sham, have had a presence for years, according to the SOHR.
Syrian state media said rebels violated the ceasefire by shelling government-held areas in the eastern Damascus neighborhood of Qaboun, wounding three people.
The Observatory said three rebel fighters and four members of the government 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 300,000 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.
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