Beirut: Air strikes by a US-led coalition have caused a "staggering" loss of civilian life in recent months around the Islamic State group's Syrian stronghold of Raqqa, a United Nations investigative body said Wednesday.

A US-backed ground force entered the city with the help of coalition air raids last week, three years after the area became a hub from which IS leaders planned expansion throughout the region and attacks around the world.

An image distributed by IS's propaganda agency Amaq shows a Raqqa mosque damaged by the coalition earlier this month. Credit:AP

Paulo Sergio Pinheiro, chairman of the UN Independent International Commission of Inquiry on Syria, said Wednesday that coalition air strikes have deepened the suffering in the city held by extremist fighters.

"We note in particular that the intensification of air strikes, which have paved the ground for an SDF advance in Raqqa, has resulted not only in staggering loss of civilian life, but has also led to 160,000 civilians fleeing their homes and becoming internally displaced," Pinheiro told the UN Human Rights Council in Geneva. He referred to the Syrian Democratic Forces (SDF), a US-backed militia dominated by Syrian Kurds.