The UN has suspended all aid convoys in Syria after a devastating attack on its lorries near Aleppo on Monday.
The strikes, which witnesses say came from the air, came hours after Syria declared a week-old US-Russia brokered cessation of hostilities at an end.
Russia and Syria have both insisted that their forces were not involved.
But UN chief Ban Ki-moon launched a stinging attack on the Syrian government, saying it had killed the most civilians in the civil war.
In unusually blunt language, Mr Ban said “powerful patrons… feeding the war machine, also have blood on their hands”.
US Secretary of State John Kerry insisted the ceasefire deal was “not dead”, following talks on the sidelines of the UN General Assembly on New York on Tuesday.
UN Syria envoy Staffan de Mistura also said there was still hope but said delegates from the Syria Support Group, which includes Russia, had agreed it was in danger.
A media activist who witnessed the attack told the BBC Arabic service that Russian reconnaissance planes had been spotted, apparently filming the passage of the convoy.
He said the first strike came at about 19:00 local time on Monday, when a helicopter dropped several barrel bombs. This was followed by rocket and machine-gun fire from aircraft, he said.
Aid workers in shock: Imogen Foulkes, BBC News, Geneva
Just a day ago, aid workers in Geneva were “almost celebrating” one said, because all the necessary permits had been received, all the warring parties had been notified, and a convoy was finally going to Aleppo province.
The 31 lorries were carrying supplies for 78,000 people in Urum al-Kubra.
But this morning, the optimistic mood had changed.
Aid workers familiar with some of the world’s most brutal conflicts were in shock, some close to tears, others expressed disgust. Vitamins and blankets, books and pencils for children, medicines to treat burns or diabetes have all been destroyed.
The head of the Syrian Arab Red Crescent’s office in Urum was killed, dying of his injuries as he waited hours to be evacuated. Now the UN and the Red Cross have suspended aid deliveries. Millions of Syrians who had hoped the brief ceasefire might bring some relief will, a Red Cross spokesman said, go on suffering, “and they have been for years”.
Ban Ki-moon, in his final address as UN Secretary-General to the UN General Assembly in New York, said many groups had killed innocents in Syria “but none more so than the government of Syria, which continues to barrel-bomb neighbourhoods and systematically torture thousands of detainees”.
He called the attack on the aid convoy “sickening, savage and apparently deliberate” and called for those responsible to be held to account.
A UN spokesman said earlier that the convoy had received proper permits, and all warring parties – including Russia and the US – had been notified.
The President of the International Committee of the Red Cross, Peter Maurer, denounced the attack as a “flagrant violation of international humanitarian law”.
Syrian Red Crescent President Abdulrahman Attar said: “It is totally unacceptable that our staff and volunteers continue to pay such a high price because of the ongoing fighting.”
The US expressed “outrage” over the attack. Washington has said it will “reassess the future prospects for co-operation” with Russia – an ally of Syria’s government.
Aid deliveries to besieged areas had been a key part of the cessation of hostilities deal brokered last week.
The convoy was being unloaded at a Red Crescent warehouse when the attack began. The aid had been due to be delivered to people in rebel-held areas around Urum al-Kubra, who were last supplied in mid-July.
A health clinic near the warehouse was also badly damaged.
The attack appeared to signal the collapse of the latest effort by the US, which backs the rebels, and Russia to halt the violence in Syria.
The Syrian military and rebels have accused each other of widespread violations.
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