Afghanistan’s Defense Ministry condemned the incident and demanded punishment for those responsible.
NATO called it a regrettable incident and announced an investigation.
The deaths are likely to strain relations between NATO and Afghan forces at a time both are calling for a closer partnership in the fight against the Taliban.
The fighting began about 3 a.m. in the eastern Wardak province, when a joint NATO-Afghan force traded fire with another Afghan unit manning the outpost.
NATO troops then called in an airstrike, killing the four Afghans, NATO and the Afghan ministry said.
“Besides expressing heartfelt condolences to the families of the martyrs, the Afghan Defense Ministry is condemning this incident,” the Afghan statement said, adding a delegation had been sent to the area to investigate.
Provincial spokesman Shahidullah Shahid said seven Afghan soldiers also had been wounded.
NATO said the Afghans began shooting first and the joint force returned fire before calling in the airstrike.
Saturday’s incident followed the deaths Friday of two U.S. service members and one U.S. employee who were killed in eastern Afghanistan. In a statement announcing the deaths, NATO did not specify the circumstances or give further details pending an investigation.
That suggested the deaths may not have been due to hostile fire.
Also Saturday, NATO said its troops opened fire on a taxi the day before as it sped toward a patrol, ignoring warning shots.
Two civilians were killed and one was wounded in the shooting, which occurred in the Muqor district of Ghazni province.
Source: Agencies