BEIRUT (AP) — Human Rights Watch says the U.S.-led coalition fighting the Islamic State group in Syria and Iraq is endangering civilians by using artillery-delivered white phosphorous.

Steve Goose, the arms director for the New-York-based rights group, says in a statement Wednesday that "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.

HRW, citing research and media reports, refers to several incidents in Raqqa and Mosul where artillery-fired white phosphorous was used but could not determine if there were casualties. The group says the rationale for the use of the weapon is unclear as the U.S.-led coalition doesn't comment on specific incidents.

White phosphorous is designed to illuminate battlefields at night, but can cause severe and lasting burns.