The UN has affirmed 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 an US-Russian brokered truce over.
“The destination of the convoy was understood to the Syrian regime and the Russian Federation,” state department spokesman John Kirby said.
“And yet these support workers were killed inside their attempt to supply help to the Syrian people,” he added.
The Syrian Observatory for Human Rights, an UK-based monitoring group, said the attacks were carried out by either Syrian or Russian aircraft.
It added the dead included motorists and the Syrian Arab Red Crescent volunteers.
The UN aid chief Stephen O’Brien said if the “callous attack” was discovered to be purposeful it’d amount to a war crime.
At least 18 of 31 lorries in the convoy were hit, based on an 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 telephone that the lorries, which were parked at a center belonging to the Syrian Red Crescent had been hit by about five missile strikes.
Deal in uncertainty
The assault seemed to signal the failure of the latest attempt and Russia to halt the violence in Syria, with the military was over.
“We don’t know if it can be salvaged,” a senior US official told journalists on condition of anonymity.
“At this point the Russians have to demonstrate very fast their seriousness of purpose because otherwise there’ll be nothing to extend and nothing to salvage.”
After the army announcement, activists said Aleppo and the surrounding state were greatly targeted, and a correspondent with the Agence France-Presse inside Aleppo city said there was virtually nonstop bombardment.
The Syrian military and rebels had already accused each other of violations.
Russia and the US are to hold additional discussions on the Syrian situation in Nyc on Tuesday, the state section added.
Help deliveries to places that are besieged were a crucial part of the cessation of hostilities deal brokered.
Nutritional supplements were delivered to the besieged town of Talbiseh on Monday in Homs state, the Red Cross said.
But most assistance shipments were waiting to go in.
The UN said it had received government approval to reach all the besieged and difficult -to-reach areas where it sought to bring help.
But access to many areas was still constrained, it said, as a result of administrative and fighting, insecurity delays.
Officials said the strikes killed more than 60 soldiers. President Bashar al-Assad called them the “latest example of flagrant American aggression against Syrian military positions in the interests of the terrorist organisation Daesh [IS]”.