By Angus McDowall and Andrew Osborn

BEIRUT/MOSCOW (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.

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 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.

SHAKY TRUCE

Earlier on Saturday, Russia and Syrian rebels cast doubt over the prospects for the increasingly shaky ceasefire, with Moscow saying the situation was worsening and a senior insurgent warning that the truce “will not hold out”.

While the ceasefire has reduced fighting, some violence has persisted across Syria. Meanwhile, there has been little movement on promised aid deliveries to besieged areas and both sides have accused the other of bad faith.

The U.N. told Reuters aid trucks which had been expected to move to Aleppo on Sunday morning, were once again being delayed.

“Obviously the humanitarian community is very frustrated by this. We have hoped to go today with the convoys,” David Swanson, spokesman for the U.N. Office for the Coordination of Humanitarian Affairs said. “We stand ready to begin the response effort as soon as we get the green light.”

Russia’s Defence Ministry said conditions in Syria were deteriorating, adding that it believed the ceasefire had been breached 199 times by rebels and saying the United States would be responsible if it were to collapse.

After the Deir al-Zor attack, it said Moscow had told the United States to rein in the Syrian opposition and make sure it did not launch a new offensive, adding that it had informed Washington about a concentration of rebels north of Hama.

Insurgents say they only reluctantly accepted the initial deal, which they believe is skewed against them, because it could relieve the dire humanitarian situation in besieged areas they control, and blamed Russia for undermining the truce.

“The truce, as we have warned, and we told the (U.S.) State Department – will not hold out,” a senior rebel official in Aleppo said, pointing to the continued presence of a U.N. aid convoy at the Turkish border awaiting permission to enter.

Rebels have also accused Russia of using the ceasefire to give the Syrian army and allied Shi’ite militias a chance to regroup and deploy forces ready for their own offensives.

(Additional reporting by Tom Perry and Lisa Barrington in Beirut, Katya Golubkova and Andrew Osborn in Moscow, Olesya Astakhova in Bishkek, Phil Stewart in Split, Croatia, Patricia Zengerle in Washington, Michelle Nichols in New York, Humeyra Pamuk and Yesim Dikmen in Istanbul and Harry Pearl in Sydney.; Editing by Dominic Evans and Toby Chopra)