An aid convoy has reportedly been hit by Syrian government or Russian military aircraft, hours after a ceasefire came to an end.
The Syrian Red Crescent trucks were hit after a routine delivery of supplies to the beleaguered city of Aleppo where fighting for control of the city has intensified as rebel groups have made gains battling Syrian government forces.
The Syrian Observatory for Human Rights (SOHR) said at least 32 people were killed in the attack.
The Red Cross earlier said the bombing killed more than 10 people.
A statement uploaded to Facebook by Aleppo activists said: "Dozens of martyrs and wounded in Aleppo, and 20 vehicles for Red Crescent set on fire in airstrikes.
"Martyrs and wounded among civilians were reported, some of them are still stuck under the rubble, in Aleppo city and its suburbs, due to bombardment and artillery shelling."
There was no immediate comment from the government on the convoy attack which comes as the SOHR said there had been at least 35 airstrikes in and around Aleppo since a US-Russia brokered ceasefire ended earlier today.
The truce had been put under intense pressure following a series of airstrikes at the weekend and a spat between Moscow and Washington over four US-led airstrikes which mistakenly killed around 60 Syrian soldiers on Saturday.
America apologised for the bombing which occurred on a base near the eastern Deir al Zor airport, insisting their intended target was Islamic State fighters.
Riad Hijab, who has been chosen by Syrian opposition groups as co-ordinator of a negotiating body to lead future peace talks, said there was never a ceasefire in the first place over which an assessment of its success or failure could be made.
As the news of the Aleppo airstrikes was breaking, the US Secretary of State was speaking before the UN summit on refguees and migrants.
John Kerry said its vital that aid workers get the protection they need.
"We have reaffirmed the principle that the impartial aid workers ought to be granted unfettered access to deliver emergency assistance to civilians who are trapped in areas of conflict," he argued.
"We may even this afternoon have heard about an incident in Syria that flies directly contrary to that right."
