A US soldier has been killed in the eastern Afghan city of Jalalabad, reportedly after being fired upon by an Afghan soldier in an "insider" attack.
The Afghan soldier was also killed when the US troops returned fire.
The shooting erupted as a US diplomat was leaving a meeting with the provincial governor.
"Insider" attacks on Nato troops by Afghan security forces grew frequent in the final years of Nato's combat mission, which wound down in 2014.
The attacks, also known as "green-on-blue", damaged trust between Nato forces and the Afghans who had been trained by them.
The US troops involved in the latest incident are part of Nato's Resolute Support mission, which replaced combat operations last December.
An Afghan soldier killed three US contractors in late January.
The motive for the latest killing is not known and details of the clash are sketchy.
The shooting took place just after the US diplomat had left the meeting with the governor of Nangarhar province.
"Suddenly an Afghan army soldier opened fire on the US soldiers who were present in the compound," Gen Fazel Ahmad Sherzad, a Nangarhar police official, told the Associated Press.
However, other police officials quoted by Reuters news agency said it was unclear who opened fire first.
Several Afghan and US troops are also said to have been hurt.
The US embassy statement in Kabul issued a statement saying it was "aware that there was an exchange of gunfire involving Resolute Support service members".
It said none of its staff had been hurt.