A US service member was killed and another wounded in an apparent insider attack in eastern Afghanistan on Monday, the Nato-led Resolute Support mission said in a statement.
The service member, who has not been identified, was the sixth American to be killed in Afghanistan this year.
The incident came two months after a member of a US army training unit was shot dead by an Afghan soldier in the southern province of Uruzgan.
"The sacrifice of our service member, who volunteered for a mission to Afghanistan to protect his country, is a tragic loss for all who knew and all who will now never know him," said Gen. Scott Miller, who assumed command of Nato forces in Afghanistan on Sunday.
Insider attacks, often known as "green on blue" attacks in which Afghan service members or attackers wearing Afghan uniforms fire on US or coalition troops, have been a regular feature of the conflict in Afghanistan, although their frequency has diminished in recent years.
Coalition forces have also tightened security and added extra force protection measures including special "Guardian Angel" units to accompany training units.
The Resolute Support statement said the wounded service member was in a stable condition but gave no further details, saying their name would be withheld until after next of kin were notified.