Syrian or Russian warplanes bombed aid trucks near Aleppo late on Monday after a week-long ceasefire ended, killing 12 people, including Red Crescent workers, the Syrian Observatory for Human Rights and opposition activists said.
The Syrian army could not immediately be reached for comment about the attack in a rebel-held area west of Aleppo near the town of Urm al-Kubra.
An opposition activist who witnessed the attack said at least eight vehicles were destroyed together with the Red Crescent's regional aid depot where they were parked.
"As I speak to you, the brothers were trying to put out the flames, and we were subjected to four more raids ... in exactly the same place," said the activist, giving his name as Abu Shahoud. "They are still in the air," he told Reuters from the area, sending footage that showed several trailers ablaze.
A local resident told Reuters by phone that the trucks had been hit by around five missiles while parked at the centre belonging to the Syrian Red Crescent, and that the head of the centre and several others were badly injured.
The United Nations had said earlier on Monday that aid including wheat flour and health supplies for 78,000 people was to be delivered to Urm al-Kubra.
The United Nations and Red Cross confirmed the convoy had been hit. The British-based Observatory said the trucks had made a routine aid delivery organised by an international organisation to an area west of Aleppo city.
"We are deeply shocked that humanitarian workers and missions have yet again suffered from the brutality of this conflict," the International Committee of the Red Cross said in a statement emailed to Reuters.