UN claims US-led coalition air strikes in Raqqa, Syria, killed 300 civilians in March

War crimes investigators with the United Nations determined at least 300 civilians have been killed since March in U.S.-led coalition efforts in Raqqa, a northern city in Syria, Reuters reported Wednesday.

According to Reuters, U.N. investigators arrived at their assessment through interviews with survivors and witnesses in the region. The U.N. Human Rights Council had previously learned the air strikes forced 160,000 civilians from their homes.

"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," Paulo Pinheiro, chairman of the U.N. Commission of Inquiry, told reporters.

In a statement, Human Rights Watch said it was concerned with the use of white phosphorus weapons. The substance is often used to create smoke screens or provide illumination, according to HRW.

"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," HRW arms director Steve Goose said. "U.S.-led forces should take all feasible precautions to minimize civilian harm when using white phosphorus in Iraq and Syria."

White phosphorus can burn through flesh down to the bone if it comes into direct contact with human skin.

Video footage of alleged white phosphorus use has been posted online by both by residents and ISIS propaganda footage, the latter of which Mic has chosen not to cite.

In its argument against the use of the weapon, HRC cited Protocol III of the Convention on Certain Conventional Weapons, a U.N. document that prohibits attacks using air-delivered incendiary weapons in civilian areas.