Syrian or Russian warplanes bombed aid trucks near Aleppo late on Monday, killing at least 32 people after a fragile week-long ceasefire ended, the Syrian Observatory for Human Rights reported.

The airstrikes on parts of Aleppo and areas to the west held by opposition forces killed at least 32 people on Monday, including 12 people in an attack that hit a convoy of aid trucks, the monitor reported.

The Observatory said the aid trucks had made a delivery organised by an international organisation to an area west of Aleppo. The United Nations and Red Cross said they were investigating the reports.

At least 18 of 31 trucks in a United Nations and Syrian Arab Red Crescent (SARC) aid convoy in Syria were hit west of Aleppo, but the world body could not verify casualties, U.N. spokesman Stephane Dujarric said on Monday.

Dujarric said the convoy was delivering assistance for 78,000 people in the hard-to-reach town of Urm al-Kubra and that multiple sources have confirmed that the town was shelled. He said an SARC warehouse was also hit.A local resident told Reuters by phone that the trucks were hit by around five missile strikes while parked in a centre belonging to the Syrian Red Crescent in the town of Urm al-Kubra, near Aleppo. The head of the centre and several others were badly injured.

The Observatory also reported 35 strikes in and around Aleppo since the truce ended.

The monitoring group said it was not clear if the jets were Syrian or Russian. Moscow supports Bashar Assad with its air force. The Syrian regime could not immediately be reached for comment.

The air strikes appeared particularly heavy in opposition-held areas west of Aleppo, near their stronghold of Idlib province. And in eastern Aleppo, a resident reached by Reuters said there had been dozens of blasts.

"It started with an hour of extremely fierce bombing," said Besher Hawi, the former spokesman for the opposition's Aleppo city council. "Now I can hear the sound of helicopters overhead. The last two were barrel bombs," he said, the sound of an explosion audible in the background.

Abu al-Baraa al-Hamawi, a rebel commander, said the most intense bombardments had taken in place in areas west of Aleppo, the same area where the aid convoy was hit. "The regime and Russians are taking revenge on all the areas," he said.