BEIRUT (Reuters) - An air strike that hit a school sheltering displaced people near the Islamic-State held Syrian city of Raqqa killed at least 33 people, a group that monitors the war said on Wednesday.

The Syrian Observatory for Human Rights said it believed the strike was carried out by the U.S.-led coalition forces fighting Islamic State in the area.

Observatory activists counted at least 33 bodies at the site near the village of al-Mansoura, west of Raqqa, Observatory Director Rami Abdulrahman told Reuters. The air strike took place on Monday night, he said.

The Pentagon said there were no indications that a U.S.-led coalition strike near Raqqa had hit civilians, but that it would carry out further investigations.

Abdulrahman said the U.S.-led coalition has escalated its aerial campaign against Islamic State around Raqqa this month, causing numerous civilian casualties.

The nearest Islamic State installation to the site of the air strike was a religious school 3 km (2 miles) away, he said.

The Syrian government urged the United Nations to stop “the devastation and destruction that the ‘international coalition’ is committing”, state-run SANA news agency said on Wednesday.

Syria’s foreign ministry accused the U.S.-led coalition of committing a “massacre” by striking a school in the al-Mansoura village, SANA reported.

A spokesman for the U.S.-led coalition has previously said that it does everything it can to avoid civilian casualties and that it investigates those that are reported as a result of its air strikes.

The Syrian Democratic Forces (SDF), a Kurdish-led alliance of militias backed by the U.S.-led coalition, is fighting to isolate Raqqa ahead of an anticipated assault on the city, which Islamic State has used as a command node to plan attacks abroad.

The head of the YPG militia, the strongest in the SDF, said last week that the offensive to retake Raqqa would begin in early April but a spokesman for the U.S. Pentagon said no decision had yet been made.