An American special forces soldier has been killed in a combat operation against Islamic State extremists in Afghanistan, the Pentagon said Sunday.

"The soldier was mortally wounded late Saturday during an operation in Nangarhar Province" in eastern Afghanistan, Navy Capt. Bill Salvin said in a message on the official Twitter account of the NATO-led Resolute Support mission.

The soldier was a special forces operator helping Afghan forces battle ISIS militants and was killed in the Achin district of Nangarhar province.

The circumstances of the death were unclear and Salvin said more information could be released later, Reuters reported.

The Pentagon said the soldier’s identity would be released pending notification of next of kin.

The ISIS terror group has a growing presence in eastern Afghanistan, where it has battled both Afghan forces and the much larger and more powerful Taliban.

Insurgents killed at least 13 Afghan security forces in separate attacks, officials said Sunday.

Munir Ahmad Farhad, spokesman for the governor of the northern Balkh province, said a roadside bomb killed nine security forces and wounded several others the night before in the Chimtal district, where they were waging an ongoing operation against the Taliban. He said five insurgents have been killed and dozens wounded.

The Associated Press contributed to this report.