Afghan and NATO officials say a coalition air strike mistakenly killed four Afghan soldiers.
Military officials say a joint force of Afghan and coalition soldiers came under fire early Saturday morning and called in the air strike.
NATO and Afghan officials say their initial investigation after the strike indicates the small arms fire originated from an Afghan National Army outpost.
The Afghan Defense Ministry released a statement condemning the mix-up and demanded that those responsible be brought to justice.
Brigadier General Eric Tremblay, a coalition spokesman, called it a regrettable incident. He said joint forces "work extremely hard" to synchronize their operations.
The Afghan army and the International Security Assistance Force announced they will conduct a joint investigation.
Saturday's incident took place in the eastern Wardak province, where a day earlier, an Afghan interpreter shot and killed two U.S. service members at a nearby base.
A military official, speaking anonymously, said a U.S. soldier then killed the interpreter. The official said the interpreter was apparently disgruntled over job-related issues.
In a statement Friday, NATO would not comment beyond confirming the deaths of two U.S. service members and a "U.S. employee."
Officials say the incidents do not appear to be linked.
Some information for this report was provided by AP and Reuters.