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, speaking alongside his Russian counterpart Sergei Lavrov, 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 had agreed the truce deal was in danger.
The attack at Urum al-Kubra destroyed 18 of 31 lorries and killed about 20 civilians, including a senior local official of the Syrian Arab Red Crescent, named as Omar Barakat.
The UN would not confirm what type of attack took place, saying “we are not in a position to determine whether these were in fact air strikes”.
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Russian defence ministry spokesman Igor Konashenkov said that after studying video taken at the scene they “did not find any signs of munitions hitting the convoy”.
“Everything shown in the video is the direct result of a fire which mysteriously began at the same time as a large scale rebel attack on Aleppo,” he said.
The Syrian military, quoted by state media, said there was “no truth” to reports that the army had targeted the convoy.
The Syrian opposition, meanwhile, hit out at the international community for not taking action against the Syrian government for ending the ceasefire.
Riad Hijab, general co-ordinator of the main umbrella group the High Negotiation Committee, said the opposition had proof Russia and Syria were behind the convoy attack, but did not give details.
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.
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” and said it could amount to a war crime.
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 said it would “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 warehouse and a nearby health clinic were also badly damaged.
The Syrian military and rebels have accused each other of widespread violations.