Two U.S. Marine Special Operations service members were killed while accompanying Iraqi forces on a mission to eliminate an Islamic State (ISIS) stronghold in northern Iraq, the Department of Defense announced Monday.

Why it matters: It's the first time this year that U.S. troops have died in combat in the American campaign against ISIS, which began in 2014.

Details: The deaths occurred while the joint mission was on an operation to clear a mountainous cave complex heavily defended by ISIS fighters, military officials told the New York Times. The Pentagon offered little information in its news release because the troops' families had not been notified.

What they're saying: “The forces trekked through mountainous terrain and eliminated four hostile ISIS fighters who were barricaded in the caves,” Col. Myles B. Caggins III told the Times in a statement.

The big picture: The Islamic State has lost all its territory, and its leader Abu Bakr al-Baghdadi was killed in a U.S. operation in October. However, the terrorist group retains the ability to recruit new soldiers and is "still very much intact," according to Masrour Barzani, the prime minister of Iraqi Kurdistan.