The UN has suspended all aid convoys in Syria after its lorries were attacked by warplanes near Aleppo on Monday.
The convoy had received proper permits, and all warring parties - including Russia and the US - had been notified, a UN spokesman said.
Eighteen of the 31 lorries, containing wheat, winter clothes and medical supplies, were destroyed.
A senior official of the Syrian Arab Red Crescent was among a number of civilians killed.
The President of the International Committee of the Red Cross, Peter Maurer, has described the attack as a "flagrant violation of international humanitarian law".
The US has expressed "outrage" over the attack, which took place in the town of Urum al-Kubra hours after the Syrian army declared a US-Russian brokered truce over.
Washington has said it will "reassess the future prospects for co-operation" with Russia - an ally of Syria's government.
Aid deliveries to besieged areas had been a key part of the cessation of hostilities deal brokered last week.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said the attacks were carried out by either Syrian or Russian aircraft.
It said the dead included Red Crescent volunteers and drivers.
The convoy was being unloaded at a Red Crescent warehouse when it came under attack. It had been due to deliver aid for 78,000 people in rebel-held areas near Aleppo.
A health clinic near the warehouse was also badly damaged.
"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.