The UN is reporting that the coalition airstrikes led by the US is causing a “staggering” number of civilian deaths in the Syrian city of Raqqa. The city was taken by Islamist State forces in 2014 and has been cited by the IS as their “capital” and their “caliphate”.

US-backed SDF (Syrian Democratic Forces) are attempting to wrest control of the city from the IS forces trapped inside in heavy fighting.

Paulo Sérgio Pinheiro, chairman of the U.N. Independent International Commission of Inquiry on Syria, said Wednesday that coalition airstrikes have deepened the suffering in the city held by extremist fighters.

“We note in particular that the intensification of airstrikes, 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 U.N. Human Rights Council in Geneva. He referred to the Syrian Democratic Forces (SDF), a U.S.-backed militia dominated by Syrian Kurds.

The commission recorded 300 civilian deaths from coalition airstrikes in Raqqa province between March 1 and May 31, according to Karen AbuZayd, an investigator for the Commission of Inquiry on Syria.

Human rights and monitoring groups have warned for months of the rising human cost of the coalition’s air war in Syria and Iraq as Islamic State forces stake out positions in densely populated civilian areas across what remains of the group’s self-proclaimed caliphate.

On March 22, the U.N. commission recorded 200 civilian deaths at an old school building in the village of Mansoura that was sheltering displaced families from across the province.

“These figures have been corroborated by multiple witnesses,” AbuZayd said.