ALEPPO - Air strikes and clashes tested a fragile ceasefire in Syria on Friday as civilians waited for aid and tensions mounted between the United States and Russia which brokered the deal.

The UN Security Council, meanwhile, was to meet later to discuss whether to endorse the truce, billed as the "last chance" to end the five-year war.

The ceasefire has been marred by a lack of aid deliveries, sporadic violence including three civilians killed Friday and increasing friction between Moscow and Washington.

US Secretary of State John Kerry called his Russian counterpart Sergei Lavrov and condemned "repeated and unacceptable delays of humanitarian aid," spokesman John Kirby said.

Kerry told Lavrov 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.

If the truce, which began on Monday, lasts seven days and humanitarian access is granted, Russia and the US are to work together to target jihadists including the extremist Islamic State group (IS)and former Al-Qaeda affiliate Fateh al-Sham Front.

Under the 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 truce deal.

"Although the ceasefire agreement is bilateral, only one side is truly implementing it," defence ministry spokesman Igor Konashenkov said in a statement.

However, key Assad backer Russia said it was still ready to extend the truce set to expire later Friday by 72 hours.

UN Security Council members were to meet at 2130 GMT for closed-door consultations, diplomats said, after Russia's envoy to the UN said Moscow wanted a UN resolution to endorse the deal.

The Pentagon said Friday 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 the Turkish military and "vetted" Syrian rebels.

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, 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.

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 spoke of a violation of the ceasefire.

The UN 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 there as soon as possible.

But early 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.

"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 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.

Washington accused Damascus of blocking aid.

"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 Friday with the ceasefire set to dominate a meeting ostensibly about countering IS.