US-led coalition air strikes reportedly killed dozens of Syrian soldiers on Saturday, endangering a US-Russian brokered ceasefire and prompting an emergency UN Security Council meeting as tensions between Moscow and Washington escalated.
The Russian Defence Ministry said US jets had killed more than 60 Syrian soldiers in four air strikes by two F-16s and two A-10s coming from the direction of Iraq.
The Syrian Observatory for Human Rights, a British-based monitoring group with contacts across Syria, cited a military source at Deir al-Zor airport as saying at least 80 Syrian soldiers had been killed.
The United States military said the coalition stopped the attacks against what it had believed to be ISIS positions in northeast Syria after Russia informed it that Syrian military personnel and vehicles may have been hit.
The United States relayed its "regret" through the Russian government for what it described as the unintentional loss of life of Syrian forces in the strike.
Pentagon press secretary Peter Cook said in an emailed statement that Russian officials did not voice concerns earlier on Saturday when informed that coalition aircraft would be operating in the strike area.
The 15-member Security Council met on Saturday night after Russia demanded an emergency session to discuss the incident and accused the United States of jeopardizing the Syria deal.
The US ambassador to the United Nations, Samantha Power, chastised 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," Power said.
She said the United States was investigating the air strikes 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," Churkin said.
Moscow cited the strikes, which allowed ISIS fighters to briefly overrun a Syrian army position near Deir al-Zor airport, as evidence that the United States was helping the jihadist militants.
"We are reaching a really terrifying conclusion for the whole world: That the White House is defending Islamic State. Now there can be no doubts about that," the RIA Novosti news agency quoted Russian Foreign Ministry spokeswoman Maria Zakharova as saying.
Power said Zakharova should be embarassed by that claim. Churkin said Russia had no "specific evidence" of the United States colluding with ISIS terrorists.
Zakharova said the strikes threatened to undermine the ceasefire in Syria brokered by Russia, which has been aiding Syria's President Bashar al-Assad in the civil war, and the United States, which has backed some rebel groups.
Apart from the US and Russian involvement, Assad is supported by Iran and Arab Shi'ite militias, while Sunni rebels seeking to unseat him are backed by Turkey and Gulf Arab states.
All the warring parties are also sworn enemies of ISIS, whose territory extends along the Euphrates valley from the Iraqi border, including around Deir al-Zor, up to land near Syria's frontier with Turkey.
In its sixth year, the conflict has cost hundreds of thousands of lives, displaced half of Syria's pre-war population, prompted a refugee crisis in the Middle East and Europe and inspired a wave of jihadist attacks across the world.
Syria's army said the US-led strikes, which took place at around 5 pm local time (1400 GMT) were "conclusive evidence" of US support for ISIS, calling them "dangerous and blatant aggression".
The US military said in its statement that Syria was a "complex situation" but that "coalition forces would not intentionally strike a known Syrian military unit".