The U.S.-led coalition fighting ISIS militants in Syria and Iraq has caused a "staggering loss of civilian life" with airstrikes around the Islamic extremists' remaining stronghold in the Syrian city of Raqqa, a top United Nations human rights official said Wednesday.

"We note in particular that the intensification of airstrikes, which have paved the ground for an SDF (Syrian Democratic Forces) advance in Raqqa, has resulted not only in staggering loss of civilian life, but has also led to 160,000 civilians fleeing their homes and becoming internally displaced," said Paulo Pinheiro, chairman of U.N.'s Independent Commission of Inquiry on the Syrian war.

Pinheiro told the U.N.'s Human Rights Council Wednesday in Geneva that, "the imperative to fight terrorism must not, however, be undertaken at the expense of civilians who unwillingly find themselves living in areas where ISIL (ISIS) is present."

Pinherio noted that if the international coalition's backing of Kurdish and Arab allied forces proved successful, it could eventually free the civilians still trapped in ISIS-held Raqqa from the militants, including religious minority women and girls, "whom the group has kept sexually enslaved for almost three years as part of an ongoing and unaddressed genocide."

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Pinheiro provided no figure for civilian casualties in and around Raqqa allegedly caused by coaltion airstrikes.

There has been a dramatic increase in reported civilian deaths resulting from U.S. and coalition airstrikes in both Syria and Iraq since the beginning of the year.

U.S. military officials insist there has been no change in the rules of engagement for American forces operating in those two countries against ISIS, and say higher civilian casualties were always expected as the militants are increasingly squeezed into shrinking territory.

Also on Wednesday, the Human Rights Watch group accused the U.S.-led coalition of endangering civilians in and around Raqqa by using artillery-delivered white phosphorous, after reports that such weapons were used in the Syrian city.

HRW said it was not able to independently verify whether the use of the munitions resulted in any civilian casualties. The northern Iraqi city of Mosul and the northern Syrian city of Raqqa, the extremist group's de facto capital, have been under attack by different groups. The U.S. military says it uses white phosphorous in a lawful way.

White phosphorous burns at extremely high temperatures and can be used to illuminate conflict zones or obscure them with smoke. International law prohibits its use in civilian areas because of its indiscriminate effects, from starting fires to causing excruciating burns for bystanders, according to Human Rights Watch.

"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. "US-led forces should take all feasible precautions to minimize civilian harm when using white phosphorus in Iraq and Syria."

HRW, citing research and media reports, referred to several incidents in Raqqa and Mosul where artillery-fired white phosphorous was used. The group said the rationale for the use of the weapon is unclear as the U.S.-led coalition doesn't comment on specific incidents.

In a video released through its Aamaq news agency, ISIS said the U.S.-led coalition used white phosphorous over Raqqa last Thursday at dusk, when Muslims would have been breaking their Ramadan fasts.

The U.S. military refused to comment on specific allegations after last week's attack in Raqqa but said it uses white phosphorous rounds "in accordance with the law of armed conflict... in a way that fully considers the possible incidental effects on civilians and civilian structures."

HRW said U.S.-led forces in Mosul and Raqqa are using U.S.-made M825-series 155mm artillery projectiles containing 116 felt wedges impregnated with white phosphorus, which ignites and continues to burn when exposed to the air. This is the only type of 155mm white phosphorus projectile in U.S. stocks that can be air-burst, HRW said.

Neither ISIS nor Syrian government forces are known to possess or to have used the U.S.-made munitions, it said.