The United States has apologised for an air strike against Syrian government forces fighting the Islamic State group.
A Russian defence ministry spokesman said 62 Syrian soldiers had been reported killed in the US-led coalition air strike on a military base.
The United Nations Security Council has scheduled a closed emergency meeting at Russia's request to discuss the incident.
A senior Obama administration official said the US has "relayed our regret" for the unintentional loss of life of Syrian forces.
The official said the notification was sent through Russia, and that the US will continue to pursue compliance with a cessation of hostilities as it continues military action against IS and an al-Qaida-affiliated group in Syria.
US Central Command said the strike was immediately halted "when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military".
US Ambassador to the United Nations Samantha Power also expressed regret for the loss of life, but also accused Russia of pulling a "stunt" by calling for an emergency Security Council meeting over the incident.
She said the US is investigating the incident in remarks made outside the Security Council chamber on Saturday night, as Russian Ambassador Vitaly Churkin was condemning the US air strike inside the closed meeting.
Ms Power told reporters: "Even by Russia's standards tonight's stunt - a stunt replete with moralism and grandstanding - is uniquely cynical and hypocritical."
She noted that Russia had never called an emergency consultation to condemn Syrian President Bashar Assad's regime "for some of the most systematic atrocities we have seen in a generation" - striking civilian targets, preventing humanitarian aid from reaching starving people, using chemical weapons or torturing tens of thousands of prisoners.
And she said a Russian foreign ministry spokesman should be "embarrassed" for issuing a statement suggesting that the air strike indicated that the US was complicit in trying to help IS.
Mr Churkin said the air strike had put "a very big question mark" over the future of the US and Russian-brokered ceasefire agreement in Syria.
He walked out of the acrimonious closed emergency session of the Security Council shortly before Ms Power began to speak.
Mr Churkin said that in his decades as a diplomat he had "never seen such an extraordinary display of American heavy-handedness as we are witnessing today".
He said the timing of the air strike is "frankly suspicious" because it comes just two days before the US and Russia were supposed to implement an agreement on military co-ordination in Syria and the US had not previously targeted IS forces fighting Syrian government troops.
Mr Churkin said if Ms Power's actions were any indication of Washington's possible reaction then the ceasefire agreement is "in serious trouble".
But he expressed hope that the US will find a way to convince Russia that it is serious about finding a political solution in Syria and fighting terrorism.