Fighters run down a street past a rising plume of smoke from a burning tire meant to disrupt warplanes in Tayyibat al-Imam, in Aleppo province, Syria, on Wednesday. (Omar Haj Kadour/Agence France-Presse via Getty Images)

— The U.S. military said Wednesday that it was investigating claims that it had bombed a school in northern Syria where activists said dozens of civilians had been killed.

The strike, which hit a building in the rural north of Raqqa province, marked the second time in a week that U.S. forces have been accused of causing mass civilian casualties in an air campaign against extremist groups.

U.S.-backed troops are also battling Islamic State militants on the ground, after the extremist group made the city of Raqqa its de facto capital during the summer of 2014.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said at least 33 bodies had been pulled from the rubble at the school, which activists from the area said had been used to house at least 50 families fleeing ­violence elsewhere in the country.

Pentagon spokesman Eric Pahon said the United States had been active in the area and was examining whether it was responsible for the strike. But separately, Col. Joseph Scrocca, a spokesman for the U.S.-led anti-Islamic State coalition, said he had received “no indication” that the bombing raid had killed civilians. “Coalition forces work diligently to be precise in our airstrikes,” he said.

Dozens of people were killed last week in U.S. strikes on al-Qaeda targets that hit a prayer hall in the town of Jinah, in Aleppo province.

The Pentagon said scores of al-Qaeda militants were killed in the attack, but residents reported that the dead were civilians who had gathered for a religious class.

On Wednesday, the Pentagon said a formal investigation had been opened into the incident.

“We are aware of the claims of civilian casualties,” said Maj. Adrian Rankine-Galloway, a Pentagon spokesman.

The disputed airstrikes come as the White House considers lifting rules put in place by President Barack Obama’s administration to avoid civilian deaths, foreshadowing a more aggressive approach to dealing with terrorist threats overseas.

In a significant development in the fight against Islamic State forces in Syria, U.S. aircraft on Wednesday helped open a new front in the campaign to recapture Raqqa.

The Pentagon said it had airlifted a coalition of Kurdish and Arab fighters known as the ­Syrian Democratic Forces, or SDF, behind Islamic State lines outside Raqqa.

Pahon said the airlift was part of a large, high-priority offensive to secure the area around the city of Tabqa and the nearby Tabqa Dam on the Euphrates River.

“This is a significant strategic target,” he told reporters. If successful, the operation could leave Islamic State militants inside Raqqa city entirely cut off from their supply lines, a tactic U.S.-backed forces have used during operations to recapture major cities from the extremist group in Iraq. 

But fighting near the Tabqa Dam is not without risks. The United Nations has previously warned of catastrophic flooding if airstrikes or Islamic State sabotage caused its water levels to rise.

Underscoring just how knotty the politics surrounding the battle for Raqqa have become, the SDF said Wednesday that the new operation also aimed to block any advance by Syrian government forces, now stationed to the west in an apparent attempt to retake the city themselves.

Thomas Gibbons-Neff in Washington and Zakaria Zakaria in Istanbul contributed to this report.