Intensified coalition air strikes have killed at least 300 civilians in the Syrian northern city of Raqqa since March, as U.S.-backed forces close in on the stronghold of Islamic State forces, U.N. 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.

"Coalition air strikes have intensified around the city," said Paulo Pinheiro, chairman of the U.N. Commission of Inquiry.

"As the operation is gaining pace very rapidly, civilians are caught up in the city under the oppressive rule of ISIL, while facing extreme danger associated with movement due to excessive air strikes," he told reporters.

Karen Abuzayd, an American commissioner on the independent panel, said: "We have documented the deaths caused by the coalition air strikes only and we have about 300 deaths, 200 in one place, in al-Mansoura, one village."

The U.N. investigators do not have access to Syria. They interview survivors and witnesses in neighboring countries or by Skype with those still in Syria.

Pinheiro, speaking earlier to the U.N. Human Rights Council, said that there had been a "staggering loss of civilian life" due to coalition air strikes that had forced 160,000 civilians to flee their homes.

Rival forces are racing to capture ground from Islamic State around Raqqa, and 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.

White phosphorus is not banned as a chemical weapon and can legally be used on battlefields to make smoke screens, generate illumination, mark targets or burn bunkers and buildings. But it can cause serious burns and start fires.

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.