A United States service member was killed by an improvised explosive device during a counter-terrorism mission against the Islamic State in Afghanistan Tuesday, according to a report by the Washington Post. It was the third time a service member was killed there this year.

Defense officials confirmed the service member’s death with the Post and said it occurred in the Nangarhar Achin province, located in the northeast region of the country and a few miles from Afghanistan’s eastern border with Pakistan.

“On behalf of all of U.S. Forces – Afghanistan, we are heartbroken by this loss and we extend our deepest sympathies to the families and friends of the service member,” Gen. John W. Nicholson Jr. said in a statement to The Post. “Despite this tragic event, we remain committed to defeating the terrorists of the Islamic State, Khorasan Province and helping our Afghan partners defend their nation.”

The service member’s name, as well as rank and their military branch, were not immediately released but defense officials said an investigation was underway.

Khorasan is considered to be the Islamic State group’s arm in the Afghanistan and Pakistan tribal region. However, ABC News reported Monday that the former al-Qaeda group is believed to have moved to Syria, where the militant group commonly known as ISIS is centrally located and caused havoc for the last three years.

The report said Khorasan was joining with al-Nusra Front, previously called al-Qaeda in Syria, to conduct coordinated attacks and that U.S. forces had targeted one of its leaders with an airstrike.

Since the U.S. invaded Afghanistan in 2001 following the Sept. 11 terrorist attacks, more than 2,300 U.S. serviceman have been killed, according to iCasualties.org.

In July, President Barack Obama said he planned to keep 8,400 U.S. troops in Afghanistan until his final term in office ends, a number much higher than his original plan of 5,500.