By Angus McDowall and Andrew Osborn

BEIRUT/MOSCOW (Reuters) - 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 U.N. Security Council meeting.

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.

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 diplomats said the 15-member body was due to meet behind closed doors at 7:30 p.m. ET.

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 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 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 (10.00 a.m. ET) 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.

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.

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.

OVERNIGHT SHELLING

Both sides have accused the other of being responsible for aid deliveries being stuck far from Aleppo, where army and rebel forces were supposed to pull back from the Castello Road which leads into besieged, insurgent-held eastern districts.

Russia on Friday said the Syrian army had initially withdrawn but returned to its positions after being fired on by rebels, who in turn say they saw no sign of government forces ever leaving their positions.

"There is no change," Zakariya Malahifji, an official for a rebel group in Aleppo, said on Saturday when asked whether there had been any move by the army to withdraw from positions along the road.

Syria's government said it was doing all that was necessary for the arrival of aid to those in need in all parts of the country, particularly eastern Aleppo.

Two convoys of aid for Aleppo have been waiting at the Turkish border for days. The U.N. has said both sides in the war are to blame for the delay of aid to Aleppo, where neither has yet withdrawn from the Castello Road.

The government said the road was being fired on by rebels, so it could not give convoys a guarantee of safety. The rebels deny the accusation.

Senior U.N. officials have accused the government of failing to provide letters to allow convoys to reach other besieged areas in Syria.

(Additional reporting by Tom Perry 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 and Humeyra Pamuk in Istanbul; Editing by Dominic Evans, Paul Simao and Mary Milliken)