BEIRUT, June 14 (Reuters) - Human Rights Watch expressed concern on Wednesday about the use of incendiary white phosphorous weapons by the U.S.-led coalition fighting Islamic State in Iraq and Syria, saying it endangers civilians when used in populated areas.

A campaign to oust Islamic State militants from the northern Syrian city of Raqqa began last week by the Syrian Democratic Forces (SDF), a group of Kurdish and Arab militias supported by a U.S.-led coalition.

Iraqi forces also supported by the coalition have been trying since October to oust the militants from Iraq's Mosul city, the other main city controlled by Islamic State.

White phosphorus munitions can legally be used on battlefields to make smoke screens, generate illumination, mark targets or burn bunkers and buildings.

Because it has legal uses, white phosphorus is not banned as a chemical weapon under international conventions, but it can cause serious burns and start fires.

"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.

"U.S.-led forces should take all feasible precautions to minimise civilian harm when using white phosphorus in Iraq and Syria," he said.

Also on Wednesday in Geneva, United Nations war crimes investigators said intensified coalition air strikes supporting an assault by U.S.-backed forces on Islamic State's stronghold of Raqqa in Syria were causing a "staggering loss of civilian life".

Human Rights Watch said it was not able to independently verify whether the use of the munitions resulted in any civilian casualties, but said it was concerned about its use in populated areas.

The international human rights organisation said white phosphorus causes severe and often fatal burns.

"White phosphorus fragments can exacerbate wounds even after treatment and can enter the bloodstream and cause multiple organ failure. Already dressed wounds can reignite when dressings are removed and they are re-exposed to oxygen," Human Rights Watch said.

U.S. Army Colonel Ryan Dillon, a spokesman for the U.S.-led coalition fighting Islamic State, said the coalition does not discuss the use of specific weapons, when asked about phosphorus use by Reuters.

"In accordance with the law of armed conflict white phosphorus rounds are used for screening, obscuring, and marking in a way that fully considers the possible incidental effects on civilians and civilian structures," Col. Dillon said in an emailed statement.

"However, ISIS continues its blatant disregard for innocent human life by killing civilians trying to flee the fighting and the Coalition will not stand idly by and allow civilians to die needlessly if we can help protect them," he said.

On Wednesday, United Nations war crimes investigators said that the intensified coalition air strikes on Raqqa were causing a "staggering loss of civilian life".

Residents as well as the campaign group Raqqa is Being Silently Slaughtered and the Syrian Observatory for Human Rights war monitor also say air strikes have killed significant numbers of civilians.

The U.S.-led coalition says it investigates any allegations of civilian deaths and is careful to avoid civilian casualties in its bombing in Syria and Iraq. (Reporting by Lisa Barrington and Ellen Francis; Editing by Tom Heneghan)