In an assault during eastern Afghanistan, two U.S. soldiers, as well as an Afghan soldier, were killed.
Nine people were injured in Saturday’s attack in Nangarhar province.
“Reports indicate an attacker opened fire on the U.S. and Afghan combined force with a machine gun in an Afghan uniform,” said U.S. military spokesman Col Sonny Leggett.
After the US invasion to oust the Taliban in 2001, about 13,000 US forces remain in Afghanistan.
“We are still gathering information and at this point, the cause or motive behind the attack is unclear,” Col Leggett said in a statement.
Sohrab Qaderi, a member of the Nangarhar provincial council, told Reuters news agency that the attacker who opened fire had also been killed during the battle.
The Afghan defense ministry said a group was investigating the incident in Nangarhar along with the US team.