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 / Updated  / Source: Reuters

WASHINGTON — A U.S. soldier was killed while conducting operations against ISIS in Afghanistan late on Saturday, a U.S. military spokesman said in a message posted on Twitter.

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

The unnamed soldier was a Special Forces operator, Salvin told Reuters separately.

He was the first U.S. soldier casualty in Afghanistan in 2017 for the Resolute Support mission, a NATO spokesman confirmed to NBC News. The circumstances of his death were unclear.

In 2016, nine American service members were killed in action in Afghanistan as part of the mission.

Related: NATO is Nervous, and Its Biggest Concern Is the White House

ISIS's offshoot in Afghanistan, known as the Sunni jihadist group's so-called Khorasan Province, is suspected of carrying out several attacks on minority Shi'ite Muslim targets. U.S. officials say intelligence suggests ISIS is based overwhelmingly in Nangarhar and neighboring Kunar province.

Estimates of its strength in Afghanistan vary. U.S. officials have said they believe the movement has only 700 fighters, but Afghan officials estimate it has about 1,500.

The Afghan Taliban, which is trying to overthrow the U.S.-backed government in Kabul, is fiercely opposed to ISIS and the two have clashed as they seek to expand territory and influence.

Chelsea Bailey contributed.