Intensified coalition airstrikes have killed at least 300 civilians in the Syrian city of Raqqa since March.
US-backed forces are closing in on the stronghold of forces belonging to the so-called Islamic State group, UN war crimes investigators have said.
The Syrian Democratic Forces, a group of Kurdish and Arab militias supported by the US-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," Paulo Pinheiro, chairman of the UN Commission of Inquiry, said.
"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 UN investigators do not have access to Syria. They interview survivors and witnesses in neighbouring countries or by Skype with those still in Syria.
Mr Pinheiro, speaking earlier to the UN 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 IS around Raqqa, and the Syrian army is also advancing on the desert area west of the city.
