The US government has expressed "anger" over an attack on an aid-convoy near the Syrian city of Aleppo that, a war monitor says, left 12 dead.
The UN has confirmed that the convoy on Monday was hit near the town of Urm al-Kubra, without giving details, reports BBC.
But activists say an airstrike hit the trucks hours after the army declared a US-Russian brokered truce over.
Meanwhile, Washington says it will "reassess the future prospects for co-operation" with Moscow.
"The destination of this convoy was known to the Syrian regime and the Russian Federation," state department spokesman John Kirby said.
"And yet these aid workers were killed in their attempt to provide relief to the Syrian people," he added.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said the attacks were carried out by either Syrian or Russian aircraft. Damascus has not commented yet.
It added the dead included the Syrian Arab Red Crescent volunteers and drivers.
The UN aid chief Stephen O'Brien said if the "callous attack" was found to be deliberate it would amount to a war crime.
At least 18 of 31 lorries in the convoy were hit, according to a UN spokesperson. The convoy was intended to deliver aid for 78,000 people.
A witness told the Reuters news agency by phone that about five missile strikes had hit the lorries, which were parked at a centre belonging to the Syrian Red Crescent.