Airstrikes carried out by a U.S.-led coalition have killed a "staggering" number of civilians in Raqqa, Syria, a United Nations investigator said Wednesday.

With the help of coalition airstrikes, U.S.-backed ground forces entered the city, Islamic State's stronghold in Syria, last week.

“We note in particular that the intensification of airstrikes, which have paved the ground for an SDF [Syrian Democratic Forces] 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 Pinheiro, the chairman of the U.N. commission of inquiry, told the human rights council in Geneva.

Karen Abuzayd, an American commissioner on the independent panel, later said investigators had documented about 300 civilian deaths caused by the airstrikes since March of this year.

Raqqa, Syria

Islamic State is known for killing civilians who attempt to flee their strongholds, and also for using civilians as human shields, which is a war crime.

U.S.-backed Syrian forces stormed into Islamic State's de facto capital of Raqqa on Tuesday, triggering intense combat and signaling the start of what the U.S.-led coalition says will ultimately be the end of IS control in the besieged city.

Raqqa was captured by IS in January 2014 and has since become the base for the group's hierarchy. The Syrian Democratic Forces (SDF) have advanced for months steadily southward toward Raqqa, clearing out IS along the way with the help of U.S. bombing efforts.