A US soldier was killed while conducting operations against Islamic State in Afghanistan late on Saturday, a US 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, said Navy Captain Bill Salvin in a message on the official Twitter account of the NATO-led Resolute Support mission.

The soldier was a special forces operator and was killed in the Achin district of Nangarhar province, Capt Salvin told Reuters separately.

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

Islamic State'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.

US officials say intelligence suggests Islamic State is based overwhelmingly in Nangarhar and neighboring Kunar province.

Estimates of its strength in Afghanistan vary.

US 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 US-backed government in Kabul, is fiercely opposed to Islamic State and the two have clashed as they seek to expand territory and influence.