A UN aid convoy delivering food relief to a rebel-held area near Aleppo was targeted in an airstrike, as a week-old ceasefire brokered by Russia and the US collapsed with a new surge of Syrian government bombing.

The US said it blamed Russia for the attack on the convoy, whether or not Russian planes were involved, arguing Moscow was responsible under the ceasefire agreement for reining in Bashar al-Assad’s government forces.

The attack on the convoy, made up of Syrian Red Crescent trucks carrying UN-supplied food, was reported to have killed at least 12 people and destroyed 18 trucks laden with food intended for tens of thousands of people cut off by the war in a rural area west of Aleppo city.

Aid officials said it was hit from the air while unloading food at a warehouse in opposition controlled Urem al-Kubra. Early reports suggested most of the dead were Syrian Red Crescent drivers.

Stephen O’Brien, the UN’s emergency relief coordinator, said that the convoy had been clearly marked and its route had been provided to all parties to the conflict.

“Let me be clear: if this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime,” O’Brien said. “I call for an immediate, impartial and independent investigation into this deadly incident. The perpetrators should know that they will one day be held accountable for violations of international humanitarian and human rights law.”

The US state department spokesman John Kirby said in a statement: “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.”

Kirby added: “The United States will raise this issue directly with Russia. Given the egregious violation of the cessation of hostilities we will reassess the future prospects for cooperation with Russia.”

“We don’t know at this point whether it was the Russians or the regime. In either case, the Russians have the responsibility certainly to restrain – refrain from taking such action themselves, but they also have the responsibility to keep the regime from doing it,” the official said.

Meanwhile, bombs and shells rained down on eastern Aleppo, home to 250,000 people cut off in an opposition-controlled area. The truce collapsed as global leaders convened at the UN in New York with the hope of consolidating the ceasefire and working on longer-term peace efforts.

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Foreign ministers from around the world in the International Syria Support Group were due to meet early on Tuesday morning before the start of the UN general debate. US officials said that the secretary of state, John Kerry, was trying to contact his Russian counterpart, Sergey Lavrov, to assess whether the ceasefire could be salvaged, but hopes of doing so were fast dimming.

A senior US official said: “At this point the Russians have the burden of demonstrating very quickly their seriousness of purpose because otherwise, as you say, there’ll be nothing to extend and nothing to salvage.”

There was no immediate public comment from Russian officials.

Hours before the convoy was struck, Kerry had pointed to the resumption of humanitarian deliveries as a sign that the ceasefire could be starting to bring benefits. But later on Monday, with the ceasefire just short of a week old, the Syrian army issued a statement blaming “terrorist groups” for hundreds of alleged violations and saying it would no longer observe the truce. The US-Russian ceasefire “was supposed to be a real chance to stop the bloodshed, but the armed terrorist groups flouted this agreement,” the statement said.

However both Kerry and the French foreign minister, Jean-Marc Ayrault, said Russia and its Syrian government allies had been responsible for the most serious violations.

“The reality, according to the information we are getting from the ground, is that violations of the ceasefire are acts of the regime,” Ayrault told reporters at the sidelines of a UN general assembly meeting in New York.

The Syrian army declaration followed the bombing of Syrian army positions around Deir ez-Zour by western coalition forces including the US, Britain, Denmark and Australia. The countries involved have said the strikes were aimed at Islamic State frontline positions and unintentionally hit Syrian troops as they fought to take those positions on Tharda mountain.

Russia has said the strikes killed 62 Syrian soldiers and injured about 100 others, and the Syrian government has described the bombing as “on purpose and planned in advance”.

The collapse of the ceasefire came just as humanitarian convoys had restarted to some of the 18 civilian enclaves cut off by the war, 15 of which are besieged by Syrian government forces and their allies, according to the UN. Aid officials had been optimistic than two convoys of 20 trucks each might finally reach 250,000 besieged in rebel-controlled eastern Aleppo.

Jan Egeland, the secretary general of the Norwegian Refugee Council and the humanitarian advisor to the UN special envoy for Syria, said the Aleppo convoys had been set to depart early on Tuesday morning.

But Egeland told the Guardian: “It hinges on so little now. What I fear we now have is a window of opportunity that we hadn’t before and we’re not able to use that window of opportunity when we have it.”