Over 100 people, and at least 42 children, have been killed in U.S.-led coalition airstrikes in eastern Syria since Thursday evening, the Britain-based Syrian Observatory for Human rights reported Friday. The strikes occurred in al-Mayadin, a town held by the Islamic State of Syria and Iraq (ISIS) near Deir al-Zor.

Two rounds of strikes, beginning Thursday evening, saw the collapse of a building that housed the families of ISIS-members, killing at least 42 children under the age of 16, the Observatory reported. The strikes also leveled a municipal building.

Many ISIS fighters and their families had fled to al-Mayadin from the city of Raqqa, an ISIS stronghold, as U.S.-backed Kurdish and Arab forces pushed towards it in an offensive against the group.

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A spokesman for the U.S.- led coalition told the Reuters news agency that that it had conducted strikes on May 25th and 26th, the results of which it was still assessing.

ISIS is losing ground in Syria and Iraq, and as its fighters retreat from other fronts, they are amassing in Syria's Euphrates basin area.