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 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.
Her opposite number, Vitaliy Churkin, said he had never seen "such an extraordinary display of American heavy-handedness" as shown by Ms 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 IS or other jihadist groups.