The U.S.-led coalition targeting the Islamic State (ISIS) is being blamed for an airstrike on a school where families had sought shelter near the northern Syrian town of Raqqa.

The monitoring group Syrian Observatory for Human Rights said at least 33 people died as a result of the Tuesday strike.

Those using the school in the village of Mansoura as shelter "were displaced civilians from Raqqa, Aleppo, and Homs," Observatory head Rami Abdul Rahman said to Agence France-Presse.

"They're still pulling bodies out of the rubble until now. Only two people were pulled out alive," he said.

The activist-run group Raqqa is Being Slaughtered Silently, which also accused U.S.-led coalition jets of being behind the airstrike, said almost 50 families were seeking refuge at the school.

The state-run Syrian Arab News Agency also reported that the U.S. led coalition was responsible for the strike.

The Associated Press writes that it "was not immediately clear who carried out the airstrike," as

Syrian Kurdish forces have been advancing on Raqqa under the cover of U.S.-led coalition airstrikes and are now 8 kilometers (5 miles) north of the city. Syrian and Russian aircraft have also carried out strikes against the IS group.

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CNN adds that ISIS-held Raqqa

is now largely surrounded, its main supply routes cut off by advancing forces. U.S.-backed Kurdish and Arab forces are squeezing ISIS from the north, while Syrian government troops—backed up by Russia—have been pushing from the west. The U.S.-led coalition against ISIS has also been carrying out airstrikes against the city.

The Pentagon said in a statement Wednesday that it has "no indication that an airstrike struck civilians near Raqqa as the Syrian Observatory for Human Rights claims," but that it would investigate. The Pentagon also said Monday that it was investigating claims that a U.S. airstrike last week killed dozens of civilians near a mosque in Aleppo, Syria.

The six years of war in Syria have killed 321,000 people, including 96,000 civilians, the Observatory said last week.

According to United Nations High Commissioner for Human Rights Zeid Ra'ad Al Hussein, the conflict is "the worst man-made disaster the world has seen since World War II," while UNICEF warned last week that Syrian children were facing "unprecedented" levels of suffering.

"Each and every child is scarred for life with horrific consequences on their health, well-being, and future," said UNICEF regional director for the Middle East and North Africa, Geert Cappelaere.