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".