Two US service members were killed and six others wounded after an attack on a joint US-Afghan operation in the Nangarhar province in eastern Afghanistan on Saturday, US forces in the country said in a statement.
The wounded service members were receiving medical treatment at a US facility.
Current reports indicate a person in an Afghan uniform opened fire on the combined US and Afghan forces with a machine gun at the Sherzad district center, said Col. Sonny Leggett, spokesman for the US forces in Afghanistan.
"We are still collecting information and the cause or motive behind the attack is unknown at this time. The incident is under investigation," Leggett said, according to the statement.
In accordance with US Department of Defense policy, the names of the service members killed will be withheld until 24 hours after notifying next of kin.
Between 12,000 and 13,000 US troops are currently serving in Afghanistan fighting terrorist groups and assisting local forces in the country.
The Trump administration is weighing further significant troop reductions in Afghanistan, despite a record high number of attacks carried out in the country last year by the Taliban and other anti-government groups.
This is a breaking news story and has been updated.