A spokesman for the U.S.-led coalition fighting Islamic State said that its forces had conducted strikes near al-Mayadin on May 25 and 26 and were assessing the results.
Two days of aerial bombardment in al-Mayadin have now killed a total of 50 people, the Britain-based Syrian Observatory for Human Rights monitoring group 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.
US has launched further airstrikes on ISIS in Syria and Iraq
Airstrikes against ISIS
Sat, February 27, 2016
British, French, Russian and US forces launch air strikes around the world.
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Russian army carried out an airstrike in Damascus
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.
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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.