US-led coalition air raids killed at least 35 civilians, including many women and children, in an eastern Syrian town held by ISIL, 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 ISIL.

“The other nine are Syrian civilians and include five children.”

Thursday’s toll brought the known deaths from two days of coalition bombing raids on Mayadeen to 50, the SOHR said, after 15 people were killed in US-led strikes on the town on Wednesday.

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

This week, it said a total of 225 civilians were killed in coalition air raids between April 23 and May 23 – the highest monthly civilian death toll for the coalition since it began bombing Syria on September 23, 2014.

The international alliance is providing air cover for twin offensives on ISIL’s remaining bastion cities: Raqqa, its self-proclaimed capital in northern Syria, and Mosul in neighbouring Iraq.

On Thursday, a Pentagon investigation concluded that at least 105 civilians were killed in an air raid on an ISIL weapons cache in Mosul in March. Reports at the time said that more than 200 people had died in the strike.

Airwars, a London-based collective of journalists and researchers that tracks non-combatant deaths in Iraq and Syria, estimated earlier this week as many as 366 civilians were killed in Iraq and Syria in April alone.

It said it had seen civilian fatalities surge since US President Donald Trump came to power and gave greater leeway to battlefield commanders.

Airwars’ figure and its claim that fatalities had risen under Trump were denied by Lieutenant General Jeffrey Harrigian, who heads US Air Forces Central Command.

Hundreds of thousands of people have been killed and millions more displaced since Syria’s conflict broke out in March 2011.