The UN has said it is ready to resume aid convoys in Syria after halting operations in response to a deadly attack on an aid mission on Monday.
Trucks will start rolling to some areas but "carefully and cautiously", the UN envoy to Syria Staffan di Mistura told the BBC on Wednesday.
Meanwhile, the US has called for all planes to be grounded in key areas of Syria to preserve the truce.
International powers will discuss the deal on Thursday in New York.
The International Syrian Contact Group includes both the US and Russia.
There was still hope for an agreement, Mistura added, saying that the US and Russia had "a responsibility".
"They have to reign in, they have to convince their own partners that this is serious. The alternative is chaos, is war. So I'm still optimistic."
Earlier at the UN, US Secretary of State 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.
US officials believe Russian aircraft were responsible for the attack, near Aleppo.
But Russia, a Syrian ally, denies being involved and its defence ministry says a US drone was in the area where the aid convoy was struck.
Russian military spokesman Gen Igor Konashenkov did not directly accuse the US of firing but pointedly said that such a drone could carry out high-precision strikes against targets on the ground.