BEIRUT (ASSOCIATED PRESS)— Syria’s military says the U.S.-led coalition carried out an airstrike on an eastern base that is surrounded by Islamic State militants, allowing the extremists to advance and potentially dealing a major blow to a shaky cease-fire.

A U.S. military official in Baghdad said he was looking into the report, which could not be independently corroborated. If true, the strike would mark the first known time the United States had targeted Syrian government forces since the start of the five-year-old conflict. It was unclear why coalition air forces would be mounting attacks during a cease-fire that the U.S has worked to put in place. However, the cease-fire does not apply to attacks on the Islamic State group.

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 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 Syrian authorities reported another 100 wounded. The planes came from the direction of the border with Iraq, he added.

He said IS militants surrounding the air base launched an attack on the Syrian army positions after the air strike. 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 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 and other terrorist groups,” using the Arabic acronym for IS. President Bashar Assad’s government views all those fighting against it as “terrorists,” and has long accused the U.S. and other rebel supporters of backing extremists.

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 cease-fire took effect on 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 the fragile 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. Officials have provided details of the agreement in press conferences, but have not released an official document, fueling suspicions on both sides.

“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.

“In my opinion, this comes from the desire to keep the combat potential in fighting the legitimate government of Bashar Assad. But this is a very dangerous route.”

He appeared to be referring to the Fatah al-Sham Front, an al-Qaida-linked group previously known as the Nusra Front, which is deeply embedded in rebel-held areas and fights alongside more moderate groups.

Russian Foreign Minister Sergey Lavrov echoed Putin’s remarks during a phone call with Secretary of State John Kerry. Lavrov noted the “refusal by an array of illegal armed groups to join the cease-fire,” and Washington’s obligation to “separate units of the moderate opposition from terrorist groupings,” according to a Foreign Ministry statement.

Under the cease-fire agreement, the U.S. and Russia would work together to target the Fatah al-Sham Front, as well as the Islamic State group, while Assad’s forces refrain from striking opposition-held areas.

But Washington has warned Russia that unless aid is delivered to Aleppo, it will not move ahead with the formation of the joint coordination center.

The U.N. has accused Assad’s government of obstructing aid access to the contested city. The Russian military says insurgents have held up the delivery by firing on government positions along the main route leading into besieged, rebel-held districts, in violation of the cease-fire.

The Syrian government said it has done all that is necessary to facilitate the entry of aid convoys to Aleppo, but that armed groups have failed to withdraw from the supply routes and are committing “dangerous, provocative acts.”

Russia’s military said Syrian rebels violated the cease-fire dozens of times over the past day, including with strikes on military and civilian targets in Aleppo.

The Interfax news agency quoted Col. Sergei Kopytsin as saying Saturday that mortar fire and homemade rockets struck Aleppo 26 times. Russian news agencies cited another official, Lt. Gen. Vladimir Savchenko, as saying there had been 55 violations throughout the country. Syria’s state news agency SANA said insurgents have violated the cease-fire 12 times in the last 12 hours. No casualties were reported.

Syrian activists said government forces have meanwhile killed five civilians. The Britain-based Syrian Observatory for Human Rights said a woman and child were killed Saturday in Talbiseh, in the central Homs province. It says two men were killed outside Damascus and a child was killed in Aleppo province.

Syria’s conflict has killed more than 300,000 people and displaced half the country’s population since March 2011.