The US-led coalition has admitted its airstrikes in eastern Syria killed at least 62 Syrian troops fighting ISIS.
Russia and Syria said the strikes prove the United States and its allies are sympathetic to ISIS.
According to the Russian military, 62 Syrian soldiers were killed near Deir Ezzor Airport. The UK-based Syrian Observatory for Human Rights put the death toll at 83 and said at least 120 soldiers were wounded.
The strike occurred September 17 in an eastern part of Syria that is not a part of a delicate and nearly week-old ceasefire. The US military said it was targeting ISIS militants and if it hit Syrian troops, it was an accident.
Hours after US-led coalition airstrikes, the US and Russian ambassadors to the United Nations chastised each other outside an emergency Security Council meeting.
The US said its planes had halted the attack in Deir al-Zour when informed of the Syrian presence.
A spokesman for the US administration expressed “regret” for the “unintentional loss of life”.
The attack caused a bitter row between the US and Russia at the United Nations Security Council.
US envoy Samantha Power accused Russia of “pulling a stunt” by calling an emergency meeting of the council.
Samantha Power’s opposite number, Vitaliy Churkin, said he had never seen “such an extraordinary display of American heavy-handedness” as shown by Power.
The Russians 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 ISIS 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 defense ministry earlier said that if the US air strikes did turn out to be an error, it would be because of Washington’s refusal to co-ordinate military action with Moscow.
Only if the current ceasefire – which began on September 12 – 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 ISIS.
The Russian defense ministry quoted a statement by Syrian army general command as saying that the four coalition air strikes on Syrian troops had allowed ISIS to advance.
The Russian foreign ministry said the attack had jeopardized the US-Russia agreement on Syria.
The Syrian statement said that the air strikes were “conclusive evidence” that the US and its allies supported the jihadist group.
There have been no confirmed cases of US air strikes targeting Syrian troops.
