The US-led coalition said on Sunday it had killed militants in eastern Syria by striking a mosque that Islamic State group used as a base, after reports of dozens of civilian deaths.

The Syrian Observatory for Human Rights said air strikes killed 41 people, including 10 children, in al-Sousa village and its environs on Thursday and Friday.

Many were Iraqi relatives of Islamic State fighters, the UK-based monitor said.

The coalition raids, which hit a mosque and houses, also killed 22 jihadists around the Euphrates River near the Iraqi border, one of the last Islamic State group enclaves in Syria.

Syria's foreign ministry said on Saturday that coalition warplanes had "committed a crime", killing at least 62 civilians in al-Sousa and a nearby village in Deir al-Zor province.

Damascus, which deems the US and international forces in Syria occupiers, accused them of relentless bombing and called on the United Nations to "punish the aggressors".

The US-led coalition said it destroyed a mosque which Islamic State fighters had turned into a headquarters in al-Sousa to defend their last territory.

The strike on Thursday killed 12 militants, disabling a command centre they had used to attack coalition forces and their allies, it said.

"Such Daesh misuse of the mosque is another example of their violation of the law of war and made the mosque a valid military target," spokesman Colonel Sean Ryan said, using the Arabic acronym for Islamic State group.

The coalition monitored the facility to know when only fighters were present, and it investigates "all credible allegations of civilian casualties", he said in an emailed statement in response to questions from Reuters.

The US-led coalition is fighting the remnants of Islamic State group near the frontier between Syria and Iraq. With air power and special forces on the ground, it backs the Syrian Democratic Forces (SDF), a Kurdish-led alliance of militias.

Various rival offensives have crushed Islamic State group in most of the territory it controlled in both countries, though fighters still operate in the desert near the border and stage attacks.