Russia said the U.S. was defending the Islamic State group after U.S.-led coalition forces allegedly bombed Syrian troops near Deir al-Zor airport on Saturday, allowing ISIS fighters to briefly overrun their position. The U.S. for its part accused Russia of grandstanding for call for an urgent UN session on the incident.

U.S.-led coalition air strikes killed dozens of Syrian soldiers on Saturday, Russia and a monitoring group said, putting a U.S.-Russian brokered ceasefire in jeopardy and prompting an emergency UN Security Council meeting.

A senior Obama administration official says the United States has "relayed our regret" for the unintentional loss of life of Syrian forces fighting the Islamic State group.

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.

Open gallery view Russian President Vladimir Putin and U.S. counterpart Barack Obama on the sidelines of the G20 Leaders Summit in Hangzhou on September 5, 2016. Credit: Alexei Druzhinin/AFP

A U.S. military official said he was "pretty sure" targets mistakenly hit in the strikes were Syrian forces.

Russia called for an emergency meeting of the United Nations Security Council after the incident, and the 15-member council met for an hour on Saturday evening.

U.S. Ambassador to the United Nations, Samantha Power, said Washington 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."

She said Russia's decision to call a council meeting was "cynical and hypocritical" as Moscow had never expressed such outrage at the killing of civilians by Syrian government forces during more than five years of conflict.

"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.

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.

She said the strikes threatened to undermine the ceasefire in Syria brokered by Russia, which has been aiding Syria's President Bashar 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 80 Syrian soldiers had been killed.

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.

All the warring parties are also sworn enemies of Islamic State, 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 U.S.-led strikes, which took place at around 5 p.m. local time (1400 GMT) were "conclusive evidence" of U.S. support for Islamic State, calling them "dangerous and blatant aggression".

The U.S. military said in its statement that Syria was a "complex situation" but that "coalition forces would not intentionally strike a known Syrian military unit."

Islamic State said via its Amaq news channel it had taken complete control of Jebel Tharda, where the bombed position was located, which would have allowed it to overlook government-held areas of Deir al-Zor.

The city's airport and some districts have been entirely surrounded by Islamic State since last year, with the airport providing their only external access.

However, Russia and Syrian state media said the Syrian army later recaptured positions it had lost. The Observatory monitoring group said at least 20 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.

Maria Zakharova, a spokeswoman for the Russian Foreign Ministry said on Saturday, said the strikes, which the Russian Defense Ministry had said killed 62 Syrian soldiers, jeopardized a Russian-U.S. agreement on Syria and looked like evidence that Washington was helping Islamic State 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 cited Zakharova as telling the Rossiya-24 TV station.

"We demand a full and detailed explanation from Washington. That explanation must be given at the UN Security Council," she said.

The United Nations Security Council will meet on Saturday at the request of Russia, diplomats said, to discuss the incident. The 15-member body is due to meet behind closed doors at 7:30 p.m. EDT (2330 GMT), diplomats said.

A U.S. military official said he was "pretty sure" the targets hit in a halted coalition-led air strike on Saturday were Syrian forces, and that the attack was carried out using U.S. intelligence after days spent tracking potential targets.

Speaking on condition of anonymity, the official said the incident had taken place at around 11 a.m. EDT (1500 GMT) and lasted about half an hour. It was halted immediately after the coalition was notified by the Russians, the official said.

The United States military said it had ceased airstrikes against what it had believed to be Islamic State positions after Russia informed it that Syrian military personnel and vehicles may have been hit.

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.

Russia, along with Iran and Arab Shi'ite militias backs President Bashar Assad, while some of the Sunni rebels seeking to unseat him are supported by the United States, Turkey and Gulf Arab states.

All those warring parties are also sworn enemies of the Islamic State jihadist group, 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.

Saturday's airstrikes were reported by Russia and a war monitor to have killed dozens of Syrian soldiers, and were said by Moscow to be evidence of what it called Washington's "stubborn refusal" to coordinate strikes with Damascus.