A US-led coalition airstrike which reportedly killed up to 80 Syrian soldiers jeopardises an American-Russian deal on Syria, says Moscow.
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".
A senior White House official said the US has relayed "regret" through the Russian government for the unintentional loss of life to Syrian forces.
The UN Security Council held an emergency meeting overnight at the request of the Kremlin.
The US ambassador to the United Nations, Samantha Power, rebuked Russia for the move.
"Russia really needs to stop the cheap point scoring and the grandstanding and the stunts and focus on what matters, which is implementation of something we negotiated in good faith with them," Ms Power said.
She said the US was investigating the airstrikes and "if we determine that we did indeed strike Syrian military personnel, that was not our intention and we of course regret the loss of life".
When asked if the incident spelled the end of the Syria deal between Moscow and Washington, Russia's UN Ambassador Vitaly Churkin said: "This is a very big question mark.
"I would be very interested to see how Washington is going to react. If what Ambassador Power has done today is any indication of their possible reaction then we are in serious trouble."
He said he had never seen "such an extraordinary display of American heavy-handedness" as displayed by Mr at the acrimonious meeting.
If confirmed, it would mark the first known direct US strike on Syrian President Bashar al Assad's forces.
The airstrike hit a base in the eastern city of Deir el-Zour, which is surrounded by Islamic State militants.
Damage caused by the strike has allowed the extremists to advance their position on to a hill overlooking the base, according to the Syrian military.
It called the strike a "serious and blatant attack on Syria and its military" and "firm proof of the US support of Daesh and other terrorist groups", using the Arabic acronym for IS.
According to a Syrian military spokesman, three tanks, three infantry fighting vehicles, four mortars and an anti-aircraft gun were destroyed, Russia's TASS news agency reported.
A US military official told Reuters news agency the strike was carried out using US intelligence, and added that the possible targets had been followed for days.
US Central Command said: "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."
But it could deal a blow to a fragile ceasefire brokered by the US and Russia which has largely held for five days, despite dozens of alleged violations on both sides.
Russian defence ministry spokesman Major General Igor Konashenkov said the airstrike was carried out by two F-16s and two A-10s. He did not identify which country in the coalition they belonged to.
He said that authorities had reported another 100 personnel were wounded, adding that if the airstrike was a mistake it was because of a "stubborn reluctance by the American side to co-ordinate its action against terrorist groups in Syria with Russia".
The defence ministry said more than 60 Syrian soldiers has been killed four airstrikes by the jets.
The Syrian Observatory for Human Rights, a British-based monitoring group with contacts across Syria, cited a military source at Deir el-Zour airport as saying at least 80 Syrian soldiers had been killed.
The ceasefire began on Monday, but aid convoys have been unable to enter rebel-held parts of the city of Aleppo - a key part of the deal.
It comes after Russian President Vladimir Putin questioned the US commitment to the ceasefire, claiming Washington was not prepared to break with "terrorist elements" battling Assad's forces.
