A Syrian war monitor said Wednesday an airstrike believed to be carried out by a U.S.-led coalition has killed at least 33 people at a school near the key city of Raqqa.
The Syrian Observatory for Human Rights reported the airstrike happened early Tuesday outside the village of al-Mansoura, and that the school was serving as a shelter for people displaced by the fighting in Syria.
Raqqa is the de facto capital for the Islamic State group, and the city's capture is a main goal for the various forces fighting the militants.
The U.S.-led coalition, which includes about a dozen countries carrying out strikes against Islamic State in Syria, has been pounding Raqqa in recent days. Coalition data showed nearly 20 strikes per day there on Saturday, Sunday and Monday.
Overall, strikes in Syria are up dramatically this year, with about 550 per month in January and February compared to an average of about 350 in the last six months of 2016.
U.S. warplanes are responsible for the vast majority of coalition airstrikes in Syria.
Those strikes, and those done by Syrian forces and their ally, Russia, have brought concerns about civilian casualties in a war that has left more than 400,000 people dead and displaced millions from their homes.
Earlier this month, the coalition released its latest civilian casualty report, saying that since operations began in August 2014 it has found "more likely than not" its airstrikes have killed at least 220 civilians. That includes strikes in both Syria and Iraq.
Rights organizations and conflict monitors say the toll is much higher.
The London-based group Airwars says coalition strikes have likely killed, at a minimum, 2,700 - 3,900 civilians in Iraq and Syria. Airwars puts the toll for Russian strikes, which have taken place only in Syria, at around 3,900 since they began in September 2015.