They recognised that “despite sporadic reports of violence, as a whole the arrangement is holding and violence is, I’d say, significantly lower in comparison to previous days and weeks,” US State Department spokesman Mark Toner told reporters.
Earlier Moscow had called for the ceasefire to be extended, despite accusing rebels of violating the truce 60 times since it came into force.
The truce, agreed after marathon US-Russia talks in Geneva last week, is part of the latest bid to end a five-year conflict that has killed more than 300,000 people.
It aims to halt fighting between President Bashar al-Assad’s forces and rebel factions, but does not include jihadists like the Islamic State group (IS).
So far it has produced “a significant drop in violence,” according to the UN’s Syria envoy Staffan de Mistura, but a key plank of the deal calling for unhindered aid access, in particular to besieged areas of Syria, has yet to be implemented.
A key focus is eastern Aleppo, where around 250,000 civilians are besieged by government forces and waiting for aid stuck on the Turkish border over security concerns.
The deal calls for the demilitarisation of the Castello Road route into the city, and Russia said Syrian troops were set to begin withdrawing by 0600 GMT on Thursday.
– UN aid trucks stranded –
But the Syrian Observatory for Human Rights, a Britain-based monitor, said Thursday that government forces were still on the road, and there was no official announcement of a withdrawal.
Moscow said Wednesday that mortar fire on the route could delay the pullback.
“It’s crucially important (that) the necessary security arrangements should be given so that they can be allowed to cross the lines,” he said.
“I have been urging the Russian government to make sure that they exercise influence on the Syrian government, and also the American side to make sure that Syrian armed groups, they also fully cooperate.”
Rebel-held eastern Aleppo is in desperate need of humanitarian aid after weeks of heavy fighting, and a government siege that has lasted most of the past two months, with no aid entering since early July.
A convoy of 20 aid trucks stocked with a month’s worth of food supplies for 40,000 people is waiting on the Turkish border, but Ban said the necessary security guarantees had not been given.
“They are at the border with Syria. They are still there,” he said.
Residents in Aleppo have welcomed the lull in the conflict that has displaced more than half their country’s population and destroyed their city, a former economic powerhouse.
But they expressed frustration about the delay in promised aid.
– Waiting for food, fuel –
“I don’t just want the renewal of the truce to be about stopping the bombing. I want them to allow in vegetables and fuel,” said 30-year-old Mustafa Morjan, in the Al-Zabdiya neighbourhood.
The deal calls for the truce to be renewed every 48 hours, and for Washington and Moscow to begin unprecedented joint targeting of jihadists like IS and former Al-Qaeda affiliate Fateh al-Sham Front if it lasts a week.
Initially, the deal allows the Syrian air force to continue strikes in areas where IS and Fateh al-Sham, previously known as Al-Nusra Front, are present.
Once the joint Russian-US targeting begins, however, government warplanes will be barred from areas where either Fateh al-Sham or opposition forces are present.
So far, the Observatory has reported minor violations by both sides, and no casualties.
But there remains deep scepticism about whether the truce will hold.
The opposition has yet to officially sign on, and hours before the ceasefire began Assad said he was committed to recovering all of Syria.
A crucial part of the deal calls on non-jihadist rebels to break ranks with Fateh al-Sham ahead of joint US-Russian operations against the group.
But many Islamist rebel groups cooperate closely with Fateh al-Sham, and the biggest of them — the powerful Ahrar al-Sham group — has criticised the terms of the Russian-US deal.
If the deal does hold, it could open the door to new peace talks to resolve the conflict, with Russia saying the UN envoy could invite government and opposition representatives to new talks “at the very beginning of October”.