The US Secretary of State has called for all planes to be grounded in key areas of Syria to save the truce there, following an attack on an aid convoy.
In a blistering speech at the United Nations, John Kerry said the future of Syria was “hanging by a thread”.
He said Monday’s attack, which killed 20 civilians, had raised profound doubt over whether Russia and the Syrian government would live up to the terms of the ceasefire deal.
Moscow has denied being involved.
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The Russian defence ministry now says a US drone was in the area where the aid convoy was struck.
Gen Igor Konashenkov said a Predator strike drone appeared above the convoy several minutes before it caught fire, and left the area 30 minutes later.
“We are not jumping to unfounded conclusions. Only its owners know why the drone was in the area at the right time and what kind of tasks it was pursuing there,” the general said.
He did not directly accuse the US of firing on the aid convoy from a drone but pointedly said that such a drone could carry out high-precision strikes against targets on the ground.
Media captionUS Secretary of State John Kerry speaking to the UN about the fragile Syria ceasefire
His comments follow Mr Kerry’s declaration that Russia should stand up and take responsibility for air strikes, criticising Russia’s defence ministry for changing its story.
He said he felt like Russia was in “a parallel universe” after listening to his Russian counterpart Sergei Lavrov address the council.
Media captionA member of the Syria Civil Defence, also known as the White Helmets, describes the aftermath of the attack on an aid convoy in Urum al-Kubra
Image copyright AFP
Image caption The UN says at least 18 trucks in the convoy were destroyed in the bombing
The UN says it has resumed preparation for convoys of aid to Syria and hopes to deliver aid to besieged and hard-to-reach areas as soon as possible.
Monday’s attack prompted the UN to suspend all aid convoys to those areas.
‘Heavy blow’
Mr Kerry said flights should stop “in order to de-escalate the situation and give a chance for humanitarian assistance to flow unimpeded”.
The attack had “dealt a heavy blow to our efforts to bring peace to Syria,” he said.
A further attack on Tuesday night killed five medical workers for an international aid agency.
A partial truce brokered by the US and Russia lasted just a week.
Heavy air raids have continued in Syria, especially in and around Aleppo.
A senior US official told the Associated Press that the US believes “with a very high degree of confidence” that the strike was carried out by a Russian-piloted aircraft.
Speaking after the UN Security Council meeting, UK Foreign Secretary Boris Johnson said “the overwhelming responsibility for the breaches in the ceasefire that we have seen lies with the Assad regime and indeed its sponsors”.
But he said, the peace process that led to the truce could be revived.
Media captionAleppo: Key battleground in Syria’s civil war
The Syrian ambassador to the UN Bashar al-Jaafari said the future of Syria would be decided by its people.
“Syria will not become another Libya or Iraq,” he said. “We will never allow this.”
A spokeswoman for the Syrian opposition delegation at the UN in New York told the BBC she was pessimistic about the future of the ceasefire.
“What are the Russians doing to enforce the regime or to pressure the regime to comply and not violate this agreement?
“The Russians themselves are violating this agreement,” she claimed.
Syria’s five-year civil war has left more than 250,000 people dead and displaced more than 11 million others.
Analysis: by Jonathan Marcus, Diplomatic correspondent, BBC News
Secretary of State John Kerry’s proposal that all aircraft should be prohibited from flying over certain areas of Syria so that humanitarian aid can be delivered unhindered probably falls short of a formal no-fly zone.
Who for example might police such a zone?
Would aircraft intruding into it be shot down, and if so by whom? Managing such small parcels of air space could also be a problem – any aircraft flying into them could be many miles away in a very short space of time.
Mr Kerry’s idea though is probably to be seen more as a declaratory policy: an attempt to get the Russians and Syrians – the only people likely to fly aircraft that might strike targets in these zones – to formally acknowledge that they will not do so
