The US has said it holds Russia responsible for a deadly attack on an aid convoy near the Syrian city of Aleppo on Monday.
The White House has called it an "enormous humanitarian tragedy".
Meanwhile, US officials have told the BBC that two Russian war planes were responsible for the attack.
Russia strongly denies involvement of its own or Syrian planes, and says the incident was caused by fire on the ground and not by an air strike.
"There are no craters and the exterior of the vehicles do not have the kind of damage consistent with blasts caused by bombs dropped from the air," a statement from the defence ministry said.
And the country's foreign ministry spokeswoman said the US government had "no facts" to support its claim, adding: "We have nothing to do with this situation."
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But US officials, speaking in condition of anonymity, said two Russian SU-24 attack aircraft were in the sky above the convoy at the precise moment it was hit in Urum al-Kubra.
The strike, they added, was too sophisticated to have been carried out by the Syrian army.
White House spokesman Ben Rhodes later said: "There only could have been two entities responsible, either the Syrian regime or the Russian government.
"In any event, we hold the Russian government responsible for airstrikes in this space."
The UN had earlier said it was "not in a position to determine whether these were in fact air strikes
