In a sign of renewed tensions between the two powers, who back opposing sides in the conflict, key regime 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 UN Security Council resolution endorsing the deal.
"We are working on it," he told reporters in New York. "I think we need to adopt it on the 21st," next Wednesday when the Security Council meets on Syria.
The UN said 20 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 on Friday.
"They've been waiting and sleeping at the border now for 48 hours. So they could go on a minute's notice," said Jan Egeland, head of the UN humanitarian taskforce for Syria.
The ceasefire deal calls for the demilitarisation 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.
Source: AFP
'Rhetorical fog'
The United States said the ceasefire was more or less holding, despite breaches by both sides, but expressed concern the UN aid convoys were blocked.
Washington said late Wednesday that US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov had agreed to prolong the ceasefire which began on 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 US is increasing the amount of attacks on residential districts."
The truce, agreed after US-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 al-Assad's forces and rebels, but excludes jihadists like the Islamic State group.
Twenty-three civilians including nine children were killed in air strikes on an IS-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.
The UN's Syria envoy Staffan de Mistura said the truce was holding "by and large".
But promised authorisation from Damascus for humanitarian convoys had not yet been received.
"This is something that is required to happen immediately," he said.
East Aleppo, where around 250,000 civilians are besieged by government forces, is a major concern for humanitarian organisations.
'Clock ticking'
The ceasefire extension "provides us a critical window of opportunity" to assist people there, said David Swanson, a spokesman for the UN 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 kilometres (44 miles) away, said Swanson.
The Observatory said earlier the army was unwilling to pull back until the opposition forces did.
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.
East Aleppo is in desperate need of humanitarian 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.
The deal calls for the truce to be renewed every 48 hours, and for Washington and Moscow to begin unprecedented joint targeting of jihadists like IS and former Al-Qaeda affiliate Fateh al-Sham Front if it lasts a week.
There remains deep scepticism about whether the truce will hold.
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.
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