People remove rubble while looking for survivors at a damaged site after an airstrike on rebel-held Idlib city, Syria March 19, 2017. (Xinhua/Reuters Photo)

DAMASCUS, March 22 (Xinhua) -- A U.S.-led airstrike against a school in northern Syria killed at least 33 people, said a monitor group on Wednesday.

The U.S.-led coalition strikes hit a school housing displaced people in the town of Mansura in Raqqa city's countryside, the Islamic State (IS) group's de facto capital, said the Syrian Observatory for Human Rights.

The UK-based watchdog group said on-ground activists said 33 bodies of slain people were dug up on Wednesday.

It said that the coalition recently increased its airstrikes against IS-held areas in Raqqa.

The U.S. coalition previously admitted there were civilian casualties due to the strikes against IS-held areas.