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WASHINGTON — Two U.S. Marines were killed in Iraq during a mission to clear out an extensive mountain cave complex where ISIS fighters were hiding and living, three military officials said Monday.

The Marines, from an elite Raiders unit, were killed by "enemy forces" as they were accompanying and advising Iraqi security forces during a raid Sunday on the stronghold of the Islamic State terrorist group in the Qara Chockh Mountains of north central Iraq, according to a statement from Combined Joint Task Force — Operation Inherent Resolve. Four other U.S. troops were wounded, the military officials said.

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The U.S. conducted airstrikes with F-15 fighter jets, Apache helicopters and drones during and after the firefight, said the officials, but the operation and the ensuing recovery of the wounded and dead lasted all day.

The names of the two service members were not immediately released pending notification of their families, the statement added.

A Kurdish Peshmerga general told an NBC News reporter visiting the area in November that ISIS was using the caves to hide as they went through the area on their way to Iraqi cities, with the intention of re-establishing a foothold in Iraq.

Around 5,200 U.S. troops are in Iraq to assist Iraqi security forces as they fight ISIS, according to The Associated Press.

Joint counter-ISIS operations between Iraq and the U.S. were restarted in mid-January after they were suspended because of a drone strike that killed Maj. Gen. Qassem Soleimani, a senior Iranian commander, in Iraq.

In February, two U.S. personnel were killed and six more were wounded while U.S. and Afghan forces were conducting a joint operation. A person wearing an Afghan uniform opened fire on them with a machine gun in Nangarhar province, Afghanistan, the military said.