Russian President Vladimir Putin listens to a journalist's question during a news conference after a CIS (Commonwealth of Independent States, former Soviet republics) summit in Bishkek, Kyrgyzstan, Saturday, Sept. 17, 2016. (Mikhail Klimentyev/Sputnik, Kremlin Pool Photo via AP)
Russian President Vladimir Putin listens to a journalist's question during a news conference after a CIS (Commonwealth of Independent States, former Soviet republics) summit in Bishkek, Kyrgyzstan, Saturday, Sept. 17, 2016. (Mikhail Klimentyev/Sputnik, Kremlin Pool Photo via AP) Credit: Mikhail Klimentyev

Beirut — The U.S. military said it might have unintentionally struck Syrian troops while carrying out a raid against the Islamic State group on Saturday, threatening an already fragile U.S. and Russian-brokered cease-fire that has largely held despite dozens of alleged violations on both sides.

If confirmed, it would mark the first known direct American strike on Syrian President Bashar Assad’s forces. The United Nations Security Council scheduled a closed emergency meeting for Saturday night at Russia’s request to discuss the airstrike.

U.S. Central Command said the strike was immediately halted “when coalition officials were informed by Russian officials that it was possible the personnel and vehicles targeted were part of the Syrian military.”

The Syrian military said the deadly airstrike hit a base in the eastern city of Deir el-Zour that is surrounded by IS, allowing the extremists to advance and overrun Syrian army positions in the area. Russia’s military said it was told by the Syrian army that at least 62 soldiers were killed in the air raid and more than 100 wounded.

The apparently errant strike could deal a crushing blow to the fragile cease-fire that has largely held for five days despite dozens of alleged violations on both sides. The cease-fire, which does not apply to attacks on IS, has already been the subject of disputes between Moscow and Washington, with each accusing the other of failing to fully implement it.

“Coalition forces would not intentionally strike a known Syrian military unit,” the U.S. military statement said.

The Syrian military said the airstrikes enabled an IS advance on a hill overlooking the air base. It called the strike a “serious and blatant attack on Syria and its military,” and “firm proof of the U.S. support of Daesh,” using the Arabic acronym for IS.

A Syrian military spokesman told a briefing early Sunday that the U.S. airstrike destroyed three tanks, three infantry fighting vehicles, four mortars and an anti-aircraft gun, Russia’s TASS news agency reported. The unnamed spokesman said the IS advance on the base was stopped after Russian warplanes were called in to hit IS positions.

Russia called for an emergency session of the U.N. Security Council and demanded “full and detailed explanations about whether this was deliberate support of the Islamic State or another mistake.”

Foreign Ministry Spokeswoman Maria Zakharova was quoted by the state news agency TASS as saying that “after today’s attack on the Syrian army, we come to the terrible conclusion that the White House is defending the Islamic State.”

A Russian Defense Ministry official said Syria has informed them that 62 of its soldiers were killed in the airstrike. Russia has been waging a year-old air campaign on behalf of Assad’s forces and closely coordinates with them.

Defense Ministry spokesman Maj. Gen. Igor Konashenkov said the airstrike near Deir el-Zour airport, held by the government, was conducted by two F-16s and two A-10s. He did not identify the planes’ country affiliation, but said they were part of the international coalition. Konashenkov said the planes came from the direction of the Iraqi border, he added.

He added that if the coalition attack was launched by mistake, the reason for it was a “stubborn reluctance by the American side to coordinate its action against terrorist groups in Syria with Russia.”

IS has repeatedly attacked the government-held air base, which is an isolated enclave deep in extremist-held territory. The government controls the air base and parts of Deir el-Zour city, while IS controls the entire province by the same name. An IS advance in Deir el-Zour would endanger the lives of tens of thousands of civilians living in government-held areas. The Syrian Observatory for Human Rights said the Syrian army later recaptured most of the positions it lost to IS.

The U.S.-led coalition has carried out thousands of airstrikes against IS in Syria and Iraq over the past two years, allowing allied forces on the ground to liberate several towns and cities from the extremist group. Russia also carries out attacks against IS targets, in Deir el-Zour and other parts of Syria.

“The coalition will review this strike and the circumstances surrounding it to see if any lessons can be learned,” the U.S Central Command statement said.

The cease-fire took effect last Monday, and despite reports of violations, it has largely held. However, aid convoys have been unable to enter rebel-held parts of the northern city of Aleppo — a key component of the deal.

Earlier on Saturday, Russian President Vladimir Putin questioned the U.S. commitment to cease-fire, suggesting that Washington wasn’t prepared to break with “terrorist elements” battling Assad’s forces.

Russia has accused Washington of failing to rein in the rebels, and on Saturday Putin asked why the United States has insisted on not releasing a written copy of the agreement.

“This comes from the problems the U.S. is facing on the Syrian track — they still cannot separate the so-called healthy part of the opposition from the half-criminal and terrorist elements,” Putin said during a trip to Kyrgyzstan.