The United Nations has expressed outrage after an aid convoy was hit in Syria, and warned the attack could amount to a war crime if it was deliberate.At least 18 trucks in the 31-vehicle convoy were hit in an attack a monitoring group said left 12 aid workers and drivers dead.
The Syrian Arab Red Crescent (SARC) trucks were hit in Urm al Kubra, a town near Aleppo, and the British-based Syrian Observatory for Human Rights said the strike was carried out by either the Syrian government or the Russian military.
It comes as ministers from the 23-nation International Syria Support Group (ISSG) meet alongside the UN General Assembly in New York to decide what next steps to take in a bid for peace.
As well as the aid convoy attack, the Syrian Observatory for Human Rights said another 20 people were believed to have been killed in separate incidents as a ceasefire brokered by the US and Russia came to an end.UN aid chief Stephen O'Brien called for an investigation into the aid convoy attack.
"Let me be clear: if this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime," he said.
The UN did not confirm the casualties cited by the Syrian Observatory for Human Rights.
Mr O'Brien said initial reports indicate that "many people" were killed or seriously injured, including SARC volunteers.
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."The US also expressed its outrage, with officials saying the attack could only have been carried out by the Syrian regime or Russia.
State Department spokesman John Kirby said: "The destination of this convoy was known to the Syrian regime and the Russian Federation."
"And yet these aid workers were killed in their attempt to provide relief to the Syrian people," he added.
"Given the egregious violation of the cessation of hostilities we will reassess the future prospects for cooperation with Russia."
US Secretary of State John Kerry will try to speak with Russia's Foreign Minister before Tuesday's meeting of the ISSG.
Mr Kerry had earlier indicated that he wants to press on with the Russian-US process, but US officials sounded more downbeat after the strike on the convoy.
The truce had already been put under 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.America apologised for the bombing which occurred on a base near Deir al Zor airport, insisting their intended target was Islamic State fighters
Prime Minister Theresa May has admitted Britain played a role in the attack while alleging the British military would never "intentionally" strike Syrian forces.
It is understood Britain's participation involved an unmanned RAF Reaper drone.
Syria's government accused rebels of more than 300 violations and failing to "commit to a single element" of the US-Russia deal.