US-led air strike in east Syria kills 35, monitor says

US-led coalition air raids killed at least 35 civilians, including many women and children as well as family members of Islamic State fighters, in an eastern Syrian town held by ISIS, according to a war monitor.

The Syrian Observatory for Human Rights (SOHR) said the strikes on Thursday hit a series of residential buildings in Mayadeen, a town in Deir Az Zor province.    

"Among the dead are at least 26 relatives of IS fighters, many of them women and children, including Syrians and Moroccans," said Rami Abdel Rahman, head of SOHR, told the AFP news agency, using a different name for ISIS.

"The other nine are Syrian civilians and include five children."    

Most air raids against Islamic State strongholds along the Euphrates basin in Syria have been conducted by a U.S.-led coalition and by Syrian and Russian jets, but Iraqi warplanes have also carried out at least one air raid in east Syria.

The deaths bring the total number of people killed in two days of aerial bombardment in al-Mayadin to 50, SOHR said.

The Britain-based monitor tracks developments in Syria's conflicts via a network of contacts on the ground.

Residents saw reconnaissance aircraft and warplanes circling the city at 7.25pm (5.25 CET) 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.

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 amassing in Syria's Euphrates basin area.

The US-led coalition against Islamic State said it is careful to avoid civilian casualties in air strikes and investigates any that are reported to have taken place.

Russia and Syria's government both deny targeting civilians.