US-led coalition airstrikes targeting Islamic State are causing a "staggering loss of civilian life" in the Syrian city of Raqqa, UN war crimes investigators say.

At least 300 people have been killed in coalition airstrikes since March, 200 of them in the village of al-Mansoura near Raqqa, investigators claim.

The coalition is supporting a ground offensive by the Syrian Democratic Forces (SDF), a Kurdish-led alliance, to retake the city from the IS miitants.

In the past week, the SDF have taken territory from the jihadists in the west, east and north of the Raqqa.

"Coalition air strikes have intensified around the city," said Paulo Pinheiro, chairman of the UN commission of inquiry.


Image: Refugees who fled from Raqqa gather at a camp in Ain Issa

"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 said.

Some 160,000 civilians are thought to have fled their homes during the conflict.

The US-led coalition estimates there are between 3,000 and 4,000 jihadists in the city, which fell under IS control in 2014.

Mr Pinheiro said if the 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".

Image: Syrian Democratic Forces soldiers gesture as they close in on the IS stronghold of Raqqa

But he added that: "The imperative to fight terrorism must not, however, be undertaken at the expense of civilians who unwillingly find themselves living in areas where [IS] is present."

Aid is struggling to get through to the Raqqa province where the UN refugee agency estimates as many as half a million people are in need of assistance.

Human Rights Watch have also warned that the coalition's use of artillery-delivered white phosphorous in Raqqa and the Iraqi city of Mosul was posing a danger to civilians.

"No matter how white phosphorus is used, it poses a high risk of horrific and long-lasting harm in crowded cities like Raqqa and Mosul and any other areas with concentrations of civilians," said Steve Goose, arms director at Human Rights Watch.

White phosphorous burns at extremely high temperatures and can be used to illuminate conflict zones or obscure them with smoke.

Its use in civilian areas is banned under international law but the US military says it has only used the chemical in a lawful way "that fully considers the possible incidental effects on civilians and civilian structures".