BEIRUT (Reuters) - Air strikes since Thursday evening have killed more than 100 people including children and other family members of Islamic State fighters in al-Mayadin, a town held by the jihadists near Deir al-Zor in eastern Syria, a war monitor reported.

The Britain-based Syrian Observatory for Human Rights said the raids were carried out by U.S.-led coalition warplanes.

A spokesman for the U.S.-led coalition fighting Islamic State told Reuters that its forces had conducted strikes near al-Mayadin on May 25 and 26 and were assessing the results.

The Observatory said more than 40 children were among those killed in the strikes, which leveled al-Mayadin’s municipality building.

Many of the families had fled from Raqqa, Islamic State’s stronghold to the northwest, which U.S.-backed Syrian Kurdish and Arab fighters are pushing toward in an offensive against the jihadists, the Observatory said.

Residents saw reconnaissance aircraft and warplanes circling the city at 7:25 p.m. (1625 GMT) before they fired missiles which struck two buildings, one of which was a four-storey block housing Syrian and Moroccan families of Islamic State fighters.

More strikes took place after midnight.

Islamic State is losing ground in both Syria and Iraq under assault from an array of sometimes rival forces in both countries. Many of its fighters who have retreated from other fronts are massing in Syria’s Euphrates basin area.

The U.S.-led coalition says it is careful to avoid civilian casualties in air strikes and investigates any that are reported to have taken place.