A joint U.S.-Afghan force clashed with Afghan troops manning a snow-covered outpost and called in an airstrike early Saturday, killing four Afghan soldiers, U.S. and Afghan officials said. Both sides called the clash a case of mistaken identity.

Afghanistan's Defense Ministry condemned the killings in the eastern Wardak province and demanded punishment for those responsible. NATO called the deaths "regrettable" 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.

Underscoring those tensions, an Afghan interpreter killed two U.S. service members Friday at a combat outpost elsewhere in Wardak province, a NATO official said.

A U.S. soldier then killed the interpreter, the official said, speaking on condition of anonymity because he wasn't authorized to release the information. NATO and Afghan officials said an investigation was still under way but the interpreter apparently was upset over job-related issues.

Both attacks occurred in the Sayed Abad district, but the official said they did not appear to be linked. First reports indicated three Americans were killed but NATO officials said one of the dead was an Afghan.

Saturday's fighting began about 3 a.m., when a joint U.S.-Afghan force traded fire with another Afghan unit manning the outpost, which the army said had been established 18 months ago to guard the highway. International troops then called in an airstrike, killing the four Afghans, NATO and the Afghan ministry said.

The NATO official confirmed they were Americans, and Afghan officials they were Special Forces working with Afghan commandos.

Associated Press Television News footage of the aftermath showed American armored vehicles on the highway, about a half mile from the hilltop outpost. The snow outside the fortified compound was blackened by the airstrike.

"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. "After the investigation is completed, the Defense Ministry wants to bring those responsible to justice."

Provincial officials said the fighting was due to a misunderstanding as the joint force returned from an operation. 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.

"We work extremely hard to coordinate and synchronize our operations," NATO spokesman Brig Gen. Eric Tremblay said.

It was believed to be the first fatal friendly fire incident since November, when eight Afghans — four soldiers, three policemen and an interpreter — were killed during close combat amid a search for a missing U.S. paratrooper.

Afghanistan's Defense Ministry said at the time that the deaths had been caused by "an air attack by NATO forces" during the fighting.

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.

U.S. soldiers shot and killed an Afghan imam Thursday when his car approached a convoy on the eastern outskirts of Kabul.

Elsewhere, joint NATO-Afghan forces came under attack in the northwestern province of Badghis at about 1 a.m. local time Saturday, prompting a gunbattle and an airstrike that killed eight militants, including a group leader, deputy provincial police chief Abdul Jabar Khan said.