Syrians deprived of food
in eastern Aleppo are still waiting for aid three days after a truce was agreed.
The UN is angry that its aid convoys remain stuck at the Turkish border because permits promised by the Syrian government have not materialised.
Russia, an ally of Syria’s president, says government forces are now moving back from a key road into Aleppo.
But a US state department spokesman said he did not have “intelligence or facts” to confirm a withdrawal.
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Getting humanitarian aid into besieged parts of the country was one of the agreements reached in the US and Russia-brokered ceasefire that began on Monday.
Some 20 lorries carrying food supplies for the estimated 250,000 people trapped in eastern Aleppo are in still in a buffer zone on the Turkish-Syrian border.
Staffan de Mistura, the UN’s special envoy to Syria, said earlier that he had not yet had “facilitation letters” from the Syrian government that would allow the convoy to pass through army checkpoints.
Russia’s deputy foreign minister Mikhail Bogdanov told the BBC that the Syrian government had concerns about the drivers of the lorries.
They “have no documents, no passport – not even driver’s licence. It’s not clear who they are,” he said, but added that “I think we can resolve this issue very quickly”.
As part of the truce agreement, both sides are expected to pull back and create a demilitarised zone around the Castello Road, considered the only route into the rebel-held east of Aleppo.
A senior Russian officer, Vladimir Savchenko, said later on Thursday that Syrian government forces had begun “a gradual withdrawal of military hardware and all personnel” – although this was denied by a rebel fighter on the ground.
State department spokesman Mark Toner said he could not confirm a pull-back by Syrian troops around Castello Road.
He said the truce was largely holding, although “we’ve seen violations on both sides”.
Washington has not commented on accusations by Russia’s defence ministry that it is failing to fulfil its obligations under the truce agreement.
Both sides have privately expressed doubts that the truce will hold.
Mr Bogdanov said “they key criteria, is action. It’s important to see how things go on the ground, and what steps are taken. That will be the real test for everyone”.
He urged the US to compel rebels to separate themselves on the ground from allied fighters from Jabhat Fateh al-Sham – a jihadist group known as al-Nusra Front until it formally broke off ties with al-Qaeda in July.
The move is necessary before the US and Russia can start conducting joint air strikes targeting Jabhat Fateh al-Sham and so-called Islamic State (IS).
Russia wants the truce deal agreed with the US to be endorsed in a resolution by the UN Security Council when it meets next week.