ALEPPO, Syria: The United Nations has urged Syria's government to allow immediate aid deliveries to hunger-stricken civilians Friday after a fragile ceasefire was extended for 48 hours by Russia and the United States.

In a sign of renewed tensions between the two powers, who back opposing sides in the conflict, key government ally Moscow accused Washington of failing to meet its obligations under the truce deal it brokered with Moscow.

Russia's ambassador to the United Nations, Vitaly Churkin, said Moscow wanted a U.N. Security Council resolution endorsing the deal.

"We are working on it," he told reporters in New York, adding that he thought it should be adopted when the Security Council meets on Syria next Wednesday.

The U.N. said trucks loaded with aid were for a second day in a buffer zone between Turkey and Syria, voicing hope the supplies could be delivered to besieged rebel-held districts of Aleppo city Friday.

The ceasefire deal calls for the demilitarization of the key Castello Road into the city.

Russia said Thursday that Syrian armed forces were "fulfilling their obligations and have started a gradual withdrawal" from the route, but rebel groups did not appear to be carrying out a simultaneous pullback as agreed.

The United States said the ceasefire was more or less holding, despite breaches by both sides, but expressed concern the U.N. aid convoys were blocked.

Washington said late Wednesday that U.S. Secretary of State John Kerry and his Russian counterpart Sergei Lavrov had agreed to prolong the ceasefire which began Monday.

But hours later Russian military spokesman Igor Konashenkov accused Washington of spinning "rhetorical fog" intended "to hide the fact that it is not fulfilling its part of the obligations".

"As of the third day (of the truce), only the Syrian army is observing the regime of silence," he said. "At the same time, the 'moderate opposition' led by the U.S. is increasing the amount of attacks on residential districts."

U.S. President Barack Obama was due to gather top national security aids Friday -- including his secretaries of state and defense -- with the shaky ceasefire set to dominate a meeting ostensibly about countering ISIS.

The truce, agreed after U.S.-Russia talks in Geneva last week, is part of the latest bid to end a five-year conflict that has killed more than 300,000 people.

It aims to halt fighting between Syrian President Bashar Assad's forces and rebels, but excludes extremists like ISIS.

Twenty-three civilians including nine children were killed in airstrikes on an ISIS-held town in eastern Deir Ezzor province Thursday, a monitor said.

The Syrian Observatory for Human Rights said it was unclear whose warplanes attacked Al-Mayadin.

Later the Observatory reported "two strikes by unknown airplanes," in rebel-held Talbisseh in central Syria.

According to the monitor, those were "the first strikes on an area where there are no extremists since the start of the ceasefire".

The U.N.'s Syria envoy Staffan de Mistura said the truce was holding "by and large".

But promised authorization from Damascus for humanitarian convoys had not yet been received.

"This is something that is required to happen immediately," he said.

- 'Clock ticking' -

East Aleppo, where around 250,000 civilians are besieged by government forces, is a major concern for humanitarian organisations.

The ceasefire extension "provides us a critical window of opportunity" to assist people there, said David Swanson, a spokesman for the U.N. Office for the Coordination of Humanitarian Affairs.

"That being said, the clock is ticking and time is of the essence."

Forty trucks carrying food for 80,000 people waited at the Turkish border to go to Aleppo about 70 kilometers (44 miles) away, said Swanson.

The Observatory said earlier the army was unwilling to pull back until the opposition forces did.

The east of Aleppo is in desperate need of aid after weeks of heavy fighting, and a government siege that has lasted most of the past two months, with no aid entering since early July.

In west Aleppo, a young boy died from his wounds after being hit by a sniper in the first death in areas covered by the truce since it started, the monitor said.

And a sniper later shot dead another person in the city's rebel-held east, it added.

The deal calls for the truce to be renewed every 48 hours, and for Washington and Moscow to begin unprecedented joint targeting of extremists like ISIS and former Al-Qaeda affiliate Fateh al-Sham Front if it lasts a week.

The opposition has yet to officially sign on, and hours before the ceasefire began Assad said he was committed to recovering all of Syria.

If the deal holds, it could open the door to new peace talks.