A man wearing an Afghan army uniform killed a U.S. service member and an American civilian in the capital Kabul on Wednesday, NATO and an Afghan official said.
A NATO statement said another U.S. service member and two U.S. civilians were wounded in the attack and were in a stable condition.
"Anytime we lose a member of our team, it is deeply painful," the statement quoted General John W. Nicholson, Commander of U.S. Forces-Afghanistan. "Our sympathies go out to the families, loved ones, and the units of those involved in this incident."
Dawlat Waziri, spokesman for the Afghan defence ministry, said the attack took place inside a military base in the capital. The ministry had launched an investigation on the incident, he added.
The NATO statement said the attacker was later killed. The victims were conducting duties as part of the larger NATO mission to train and advise the Afghan security services, and an investigation was being conducted to determine the exact circumstances of the incident.
In May, two Romanian soldiers were shot and killed when two Afghan army recruits opened fire on them in southern Kandahar province. Both attackers were later killed by other troops in a military comp.