(REUTERS) – U.S.-led coalition air strikes reportedly killed dozens of Syrian soldiers on Saturday, endangering a U.S.-Russian brokered ceasefire and prompting an emergency U.N. Security Council meeting as tensions between Moscow and Washington escalated.

The United States military said the coalition stopped the attacks against what it had believed to be Islamic State 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, a senior Obama administration official said in an emailed statement.

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 U.S. 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 told reporters.

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 U.N. 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 told reporters.

Moscow cited the strikes, which allowed Islamic State 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 embarrassed by that claim. Churkin said Russia had no “specific evidence” of the United States colluding with Islamic State militants.

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.

The Russian Defence Ministry said U.S. 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 90 Syrian soldiers had been killed.

Australia also participated in the strikes and the Australian Department of Defence offered its condolences to the families of Syrian soldiers killed or wounded in the incident.

The ceasefire, which took effect on Monday, is the most significant peacemaking effort in Syria for months, but has been undermined by repeated accusations of violations on both sides and by a failure to bring humanitarian aid to besieged areas.

Apart from the U.S. 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.

However, Russia and Syrian state media said the Syrian army later recaptured positions it had lost. The Observatory monitoring group said at least 30 Islamic State fighters were killed in heavy Russian air strikes during that fighting.

The incident also threatens to undermine proposed joint targeting by the United States and Russia of Islamic State and some other jihadist groups across Syria.