US-led coalition air strikes have killed at least 62 Syrian troops fighting so-called Islamic State in the east, the Russian military says.
Aircraft flew in from Iraq and bombed Syrian troops near Deir al-Zour airport, allowing the IS jihadists to advance, the Russians said.
The US has not yet commented.
Russia earlier expressed fears the current ceasefire in Syria could collapse, saying the US would be to blame.
The cessation of hostilities does not include attacks by the US on IS or other jihadist groups.
Russia’s defence ministry said that if the US air strikes turned out to be an error, it would be because of Washington’s stubborn refusal to co-ordinate military action with Moscow.
Only if the current ceasefire – which began on Monday – holds for seven days, will the US and Russia begin co-ordinated action against the Jabhat Fateh al-Sham group, which was previously known as the al-Nusra Front, and IS.
Russia’s defence ministry quoted a statement by Syrian army general command as saying that coalition jets had bombed a Syrian army position at Jebel Tharda near Deir al-Zour. The four strikes had allowed IS to advance.
The Syrian statement said the air strikes were “conclusive evidence” that the US and its allies supported the jihadist group.
The Syrian Observatory for Human Rights monitoring group put the death toll at at least 80.
Russian defence ministry spokesman Maj-Gen Igor Konashenkov said the attack was on Syrian troops surrounded by IS fighters and was carried out by two F-16 and two A10 aircraft flying in from Iraq.
There have been no confirmed cases of US air strikes targeting Syrian troops. Last December, Syria accused the coalition of attacking a government army camp in Deir al-Zour but the US denied it.
Earlier, Russia’s military expressed fears for the ceasefire. It said rebel groups had increased attacks and it urged the US to act or be responsible for the truce’s collapse.
Russian General Vladimir Savchenko said “the situation in Syria is worsening”, with 55 rebel attacks over the past 24 hours, leading to the deaths of 12 civilians.
Gen Viktor Poznikhir said Russia, an ally of the Syrian government, was doing all it could to rein in Syrian troops.
“If the American side does not take the necessary measures to carry out its obligations… a breakdown of the ceasefire will be on the United States,” he said.
“The United States and the so-called moderate groups they control have not met a single obligation they assumed in the framework of the Geneva agreement.”
The terms require moderate rebel groups to separate themselves from jihadists.
Gen Poznikhir said: “Our repeated messages to the American side are left without a response. There is doubt that the US is able to influence the moderate opposition they control.”
A US National Security Council spokesman later said: “While there have been challenges on both sides, violence is considerably lower and the cessation is broadly holding.
“What we’re not seeing is humanitarian aid getting through and it will be hard to build confidence on the ground until that occurs.”
Some 20 trucks have been waiting since Monday for safe passage from Turkey into Syria and on to rebel-held east Aleppo.
Truce’s days may be numbered – BBC’s James Longman, Beirut
This was meant to be a trust-building exercise, but nearly a week after the truce began, the blame game has begun.
There was deep scepticism from the rebels about details in the plan which called for their separation from extremist groups. That is why they never formally accepted the deal.
It was always a major sticking point. Were US backed groups supposed to surrender territory to Jabhat Fateh al-Sham? Or were they required to fight them? It was never clear. Either way, the stipulation would leave them far weaker on the battlefield. But refusing and standing in the way of much-needed humanitarian aid would not have been popular.
Now this weekend, the main rebel groups are due to meet to discuss their position. Their mistrust of the government and its Russian allies runs deep. They see the obstruction of aid deliveries on the border as a stalling tactic, and one which they have seen before.
If aid doesn’t reach besieged areas soon, the ceasefire’s days are numbered. And co-ordinated strikes against IS won’t happen.