Syria war: US and Russia argue over truce
There are signs of growing tensions between the US and Russia over Syria - four days after a ceasefire came into effect in the country.
Moscow warned it could resume air strikes on "moderate" rebel groups unless Washington did more to distance them from extremists.
In turn, the US voiced concern about delays in providing humanitarian aid to Aleppo and other besieged areas.
Truce breaches by Syrian troops and rebel groups have also been reported.
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'No Plan B'
If the cessation holds for seven days, the US and Russia have agreed to plan joint attacks on the Jabhat Fateh al-Sham group, which was previously known as the al-Nusra Front, and so-called Islamic State (IS).
On Friday, US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov discussed the crisis in a telephone call.
Mr Kerry warned Moscow that US-Russian joint military planning would not happen until humanitarian aid began to arrive.
Mr Kerry "emphasised that the United States expects Russia to use its influence on the Assad regime to allow UN humanitarian convoys to reach Aleppo and other areas in need", state department spokesman John Kirby said in a statement.
"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.''
The UN says it is still waiting to be able to deliver aid to besieged Aleppo.
Some 20 trucks have been waiting for safe passage to cross from Turkey into Syria and on to rebel-held east Aleppo since Monday.
However, the UN says it has not yet received permits from the Syrian government to allow the trucks into opposition areas, where at least 250,000 people are in desperate need of food and medicine.
Meanwhile, a Russian foreign ministry statement said Mr Lavrov had urged Mr Kerry to "fulfil its promise faster" in separating the moderate opposition from extremists.
Mr Lavrov also called on the US to make the ceasefire agreement public and have the UN Security Council endorse it.
Russian Deputy Foreign Minister Mikhail Bogdanov earlier told the BBC his country was confident that Syria would honour the ceasefire agreement but had "more doubts about the opposition".
The US "have been promising to do the maximum possible to separate the moderate opposition from the Nusra Front since February", when a previous cessation of hostilities deal was agreed, said Mr Bogdanov.
Since then, the US had been saying "just hold on", he said. "We're waiting, but there are limits," the Russian official added.
Mr Bogdanov said the current deal was "the only plan on the table". "We have no Plan B," he said.
Russian officials also accused US-backed opposition fighters of failing to withdraw from Castello Road on the outskirts of Aleppo - the main route to besieged rebel-held areas in the city.
Despite this, Moscow suggested that it was ready to extend the ceasefire for another 72 hours.