The UN has confirmed that the convoy was hit near the rebel-held town of Urum al-Kubra, without giving details.
Activists say an air strike hit the trucks hours after the army declared a US-Russian brokered truce over.
The US says it will “reassess the future prospects for co-operation” with Russia – an ally of Syria’s government.
“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.
- Children suffer horrors of Syria’s war
- US unease over joint air action with Russia
- How will the new truce work?
- What’s left after five years of war?
The Syrian Observatory for Human Rights, a UK-based monitoring group, said the attacks were carried out by either Syrian or Russian aircraft.
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 in rebel-held areas near Aleppo.
A witness told 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.
