The ongoing siege by Syrian Kurdish militia supported by airstrikes of the US-led coalition on terrorist stronghold Raqqa is marred by many civilian casualties and displacement of some 160,000 people, a UN commission says.

The Syrian Democratic Forces (SDF), a group of militias dominated by ethnic Kurds, launched their offensive on Raqqa last week amid weapons and military hardware supplies from the US. The effort is taking a serious toll on civilian residents of the city controlled by Islamic State (IS, formerly ISIS/ISIL), chairman of the UN Commission of Inquiry into the Syrian conflict said on Wednesday.

“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,” Paulo Sergio Pinheiro told the UN Human Rights Council, Reuters reports.

The official didn’t give an estimate for civilian casualties in the city.

The US troops deployed in Syria are supporting the SDF offensive providing artillery and air support on the battlefield as well as “advice and training” for the militias.

The Pentagon calls the deployment, which is done with no UN Security Council mandate or invitation from the Syrian government, a limited-scale operation. The Americans have attacked pro-government troops in Syria on several occasions over the past few weeks, claiming it was necessary to protect its SDF allies.

Also on Wednesday the Human Rights Watch (HRW) expressed its concern over the toll the Raqqa offensive operation is taking on civilians. The group criticized the US troops for allegedly using white phosphorous munitions as part of the offensive, saying the chemical may cause serious injuries and trigger fires when used in residential areas.

In his speech, Pinheiro also criticized Damascus for negotiating a series of evacuations of opposition fighters and their families from various areas around Syria, including the December evacuation from Aleppo. He said in some cases the residents had no choice but to leave and that their relocation may amount to a war crime.

The evacuation from Aleppo came after years of fighting for control of the city as it was split between pro-government forces and parts held by rebel. The fighting resulted in numerous deaths among residents. The evacuation was negotiated by Russia, Iran and Turkey and came as an alternative to continued street-to-street battles for Aleppo.