The Islamic State in Iraq and Syria (ISIS) used chemical weapons against the Kurdish peshmerga in August, two U.S. defense officials tell The Hill.

The officials said they have confirmed that ISIS used chemical weapons during fighting in northern Iraq and that the Kurdish military — a staunch ally of the United States — was targeted.

“We can confirm some type of chemical weapon was employed by ISIL in that August mortar attack,” a U.S. defense official told The Hill on the condition of anonymity, using another acronym for ISIS.

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A second U.S. defense official said ISIS used sulfur mustard in the attack, which took place in the city of Makhmur.

James Clapper, the nation’s top intelligence official, last week acknowledged for the first time that ISIS has used chemical weapons in Iraq and Syria but did not specify where.

“ISIL has also used toxic chemicals in Iraq and Syria, including the blister agent sulfur mustard,” the director of national intelligence said.

The U.S. military sent mortar fragments from the August attack to a lab in the U.S. for additional testing after a preliminary field test showed the presence of mustard agent.

During that August attack, ISIS launched 45 120-millimeter mortar shells tipped with chemical heads believed to contain mustard gas on peshmerga positions, according to information provided by the Kurdistan Regional Government.

A number of peshmerga suffered burns, and about 60 suffered injuries to their throats, the regional government said.

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Rep. Joe Wilson Addison (Joe) Graves WilsonDemocrats raise alarm about new US human rights priorities Democrat Teresa Leger Fernandez defeats Valerie Plame in New Mexico primary Trump campaign launches new fundraising program with House Republicans MORE (R-S.C.), chairman of the House Armed Services subcommittee on emerging threats, called ISIS’s ability to manufacture chemical weapons a “grave threat” to U.S. troops and American families at home.

“We need a strong President who has a plan to defeat ISIS and enact a strategy of peace through strength,” Wilson said in a statement to The Hill.

Clapper said at a conference in Germany on Friday that ISIS had used “industrial chemicals,” notably mustard gas, multiple times in the Middle East against adversaries and that ISIS would like to use chemical weapons against the U.S.

He did not specify whether that meant an attack on the U.S. homeland or against U.S. troops abroad.

CIA Director John Brennan spoke publicly about the issue recently, saying on CBS News’s “60 Minutes” Sunday that the CIA believes ISIS has the ability to manufacture small quantities of chlorine and mustard.

Those comments reflect the U.S.’s testing of about 10 different samples collected from the battlefield that show a rudimentary ability to manufacture chlorine and mustard into weapons, according to the second U.S. defense official.

ISIS’s usage of chemical weapons could put U.S. troops at risk.

There are currently about 3,700 U.S. troops in Iraq, including some in Makhmur, where the U.S. has a new joint operations command center with the Iraqis.

ISIS has attacked Makhmur as recently as Jan. 24.

There are also several hundred U.S. troops about 40 miles north of Makhmur, in Erbil, where the U.S. has a joint operations center. There, U.S. personnel are training, advising and assisting Kurdish peshmerga at the battalion level, which brings them closer to the battlefield than when they work with regular Iraqi army forces.

In some cases, U.S. forces are partnering with Kurdish peshmerga on combat operations. In one such raid in Hawija to rescue hostages from ISIS last October, one Army Delta Force soldier was fatally wounded.

U.S. troops could possibly get even closer as the Iraqi security forces and peshmerga prepare to take back Mosul, which is only 50 miles away from Makhmur and Erbil.

The second U.S. defense official said there is no indication ISIS is stockpiling any chemical weapons to use in the Mosul offensive.