A London-based activist and monitoring group said Wednesday that a U.S.-led coalition airstrike had hit a school in ISIS-held territory in northern Syria that was being used to shelter displaced families, killing dozens of civilians.

The Syrian Observatory for Human Rights (SOHR), which relies on an extensive network of contacts on the ground in Syria and which generally proves a reliable source of information on the war, said coalition aircraft “most likely” carried out the strike but did not explain how it reached that conclusion.

In a statement distributed later Wednesday, the U.S.-led coalition said it had no evidence yet to back up the claim of a strike hitting civilians, but it acknowledged airstrikes in the vicinity ahead of a first-of-its-kind airdrop of rebel fighters by American helicopters.

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The statement said the unprecedented ferry service for about 500 U.S.-trained fighters “was preceded by synchronized Coalition fires throughout the area. I cannot say if the alleged civilian casualty was due to this, however, since we have conducted several strikes near Raqqa we will provide this information to our civilian casualty team for further investigation.”

The coalition has targeted ISIS militants and infrastructure in and around the northern Syrian city of Raqqa for months. It is the terror group’s last major urban stronghold in Syria and its self-declared capital.

U.S. officials confirmed earlier Wednesday to CBS News’ David Martin that American helicopters had airlifted some 500 Syrian fighters into an area west of Raqqa. It was the start of an operation intended to seize a major dam across the Euphrates River, at Tabqah, which is currently held by ISIS and is considered one of the few remaining ways of getting supplies and people in and out of Raqqa.

It was the first known instance of American helicopters ferrying Syrian fighters into battle. The Syrians were trained and equipped by U.S. special operations forces, but the statement from the U.S.-led coalition said no American advisers were accompanying the fighters on the ground.

A Google Map shows the location (with red marker) of the town of Mansour, between the ISIS-held city of Raqqa and Tabqah to the west, where a crucial dam spans the Euphrates river, providing access to central Raqqa from surrounding areas. Google Maps

The school that was destroyed by the airstrike is located between Raqqa and the Tabqah dam.

Russian aircraft have also carried out one or two strikes in the area in recent months, but have largely focused their attention elsewhere, targeting groups other than ISIS. Both U.S. and Russian-backed factions on the ground are battling ISIS in an effort aimed at eventually retaking Raqqa.

The Wednesday strike reportedly hit a school in the town of Mansoura, about 15 miles southwest of central Raqqa.

“We can now confirm that 33 people were killed, and they were displaced civilians from Raqqa, Aleppo and Homs,” SOHR director Rami Abdel Rahman told the French news agency AFP. They’re still pulling bodies out of the rubble until now. Only two people were pulled out alive.”

A second activist and monitoring group, “Raqqa is Being Slaughtered Silently,” also reported the airstrike on the school, saying as many as 50 displaced families were being sheltered in the building.

ISIS is known to position its fighters and supplies near to, and even in the same buildings as civilians, in an effort to deter airstrikes. Activists said ISIS militants were at the scene of the strike on the school Wednesday within minutes, ushering people away from the building.

The reports about the strike in Mansoura come less than a month after the same monitoring groups said a U.S.-led coalition aircraft had hit a village just east of Raqqa, killing at least 20 civilians.

About 200 U.S. Marines deployed into Syria with heavy artillery in early March as part of preparations for the fight to oust ISIS from Raqqa. Pentagon officials said that was a long-planned increase in presence, and not part of an anticipated ramping-up of operations against ISIS that President Trump and his senior military commanders are still hashing out.