Soldiers patrol near destroyed buildings in Raqqa, the Syrian city that was a stronghold of the Islamic State. (Youssef Rabie Youssef/European Pressphoto Agency)

 Airstrikes by a U.S.-led coalition have caused a “staggering” loss of civilian life in recent months around the Islamic State’s Syrian stronghold of Raqqa, a United Nations investigative body said Wednesday.

A U.S.-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 Islamic State leaders planned expansion throughout the region and attacks around the world.

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.

The U.S. military said at the time that it was aware of the reports and was opening an investigation.

The activist group Raqqa is Being Slaughtered Silently, which monitors violence in the province, provided The Washington Post with the names of 40 people who it said were killed at the school in Mansoura.

A representative from the group said the bodies of dozens more people without identification cards were believed to have been buried in mass graves.

The U.N. refu­gee agency made a public plea this week for better access to the province, where tens of thousands of civilians need assistance amid the fighting.

The U.S.-led coalition includes military personnel from dozens of countries, among them Britain, France and Australia. According to its latest report, at least 484 civilians have been unintentionally killed by coalition airstrikes since June 2014.

But the Britain-based tracking group Airwars puts that figure eight times higher, claiming that more than 3,800 people were killed in that period.

Although the nonprofit group previously tracked casualties from both U.S.-led and Russian strikes, it said it now is concentrating resources on claims relating to the U.S.-led coalition to keep up with their pace.

Fatalities from coalition bombing raids now outstrip those caused by Moscow’s warplanes, according to Airwars.

The U.S. military said Tuesday that it has added five full-time members to a team that monitors civilian casualty claims. The team previously consisted of two full-time and two part-time personnel. Army Col. Ryan Dillon said that aviation, intelligence and legal experts were added to help boost the team’s response time on civilian casualty reports.

At the U.N. Human Rights Council on Wednesday, Pinheiro warned that the fight against Islamic State forces must not be undertaken at the “expense of civilians who unwittingly find themselves living in areas” where the group is present.