The US-led coalition has admitted its planes carried out an attack in eastern Syria that the Russian army says killed at least 62 Syrian troops fighting IS.
The US said its planes halted the attack in Deir al-Zour when informed of the Syrian presence and would not knowingly strike them.
The strikes allowed IS jihadists to advance, the Russians said.
Russia earlier said the current ceasefire in Syria was in danger of collapse and the US would be to blame.
The cessation of hostilities does not include attacks by the US on IS or other jihadist groups.
The US Central Command statement said the coalition believed it was attacking positions of so-called Islamic State and the raids were “halted immediately when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military”.
It said the “Combined Air Operations Center had earlier informed Russian counterparts of the upcoming strike”.
It added: “Syria is a complex situation with various military forces and militias in close proximity, but coalition forces would not intentionally strike a known Syrian military unit. The coalition will review this strike and the circumstances surrounding it to see if any lessons can be learned.”
Russia’s defence ministry earlier said that if the US air strikes did turn 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 the four coalition air strikes on Syrian troops 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.
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 collapse of the truce.
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.