Six civilians, including a child, were among at least 22 people killed in US air strikes on the stronghold of the Takfiri group, ISIL (so-called Islamic State in Iraq and Syria), on Saturday and Sunday, a monitor said.

Six civilians, including a child, were among at least 22 people killed in US air strikes on the stronghold of the Takfiri group, ISIL (so-called Islamic State in Iraq and Syria), on Saturday and Sunday, a monitor said.

The rest of the dead in the raids on the city of Raqa were ISIL militants, the Syrian Observatory for Human Rights said.

Dozens of people were also wounded in the raids which took place on Saturday and Sunday.

The US-led coalition, which launched an air war against ISIL in Syria last September, said it had carried out "significant" air strikes against Raqa.

"The significant air strikes tonight were executed to deny Daesh (ISIL) the ability to move military capabilities throughout Syria and into Iraq," spokesman Lieutenant Colonel Thomas Gilleran said.

"This was one of the largest deliberate engagements we have conducted to date in Syria and it will have debilitating effects on Daesh's ability to move from Raqa."

Coalition forces "successfully engaged multiple targets" throughout Raqa, the statement said, destroying IS structures and transit routes.

The strikes "have severely constricted terrorist freedom of movement," it added.

Washington is leading an international coalition fighting ISIL in Syria and Iraq, where the extremist group has proclaimed an Islamic "caliphate" in territory under its control.

However, many reports make doubts about the real intentions of the US raids, pointing out that the coalition warplanes had dropped weapons and several supplies to the Takfiri insurgents.

Leader of the Islamic Revolution in Iran, Imam Sayyed Ali Khamenei had repeatedly accused the US of backing the Takfiris, saying that Washington is not honest in fighting ISIL.