By Stephanie Nebehay

GENEVA (Reuters) – Intensified coalition air strikes supporting an assault by U.S.-backed forces on Islamic State’s stronghold of Raqqa in Syria are causing a “staggering loss of civilian life”, United Nations war crimes investigators said on Wednesday.

The Syrian Democratic Forces (SDF), a group of Kurdish and Arab militias supported by a U.S.-led coalition, began to attack Raqqa a week ago to take it from the jihadists. The SDF, supported by heavy coalition air strikes, have taken territory to the west, east and north of the city.

“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,” Paulo Pinheiro, chairman of the U.N. Commission of Inquiry told the Human Rights Council.

Pinheiro provided no figure for civilian casualties in Raqqa, where rival forces are racing to capture ground from Islamic State. The Syrian army is also advancing on the desert area west of the city.

Separately, Human Rights Watch expressed concern in a statement about the use of incendiary white phosphorous weapons by the U.S.-led coalition fighting Islamic State in Iraq and Syria, saying it endangered civilians when used in populated areas.

In its speech to the 47-member forum in Geneva, the U.S. delegation made no reference to Raqqa or the air strikes. U.S. diplomat Jason Mack called the Syrian government “the primary perpetrator” of egregious human rights violations in the country.

Pinheiro said that if the international coalition’s offensive is successful, it could liberate Raqqa’s civilian population, including Yazidi women and girls, “whom the group has kept sexually enslaved for almost three years as part of an ongoing and unaddressed genocide”.

“The imperative to fight terrorism must not, however, be undertaken at the expense of civilians who unwillingly find themselves living in areas where ISIL is present,” he added.

Pinheiro also said that 10 agreements between the Syrian government and armed groups to evacuate fighters and civilians from besieged areas, including eastern Aleppo last December, “in some cases amount to war crimes” as civilians had “no choice”.

Syria’s ambassador to the U.N. in Geneva, Hussam Edin Aaala, denounced violations “committed by the unlawful U.S.-led coalition which targets infrastructure, killing hundreds of civilians including the deaths of 30 civilians in Deir al-Zor.”

(Reporting by Stephanie Nebehay; Editing by Tom Miles and Tom Heneghan)