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Kabul, Afghanistan (CNN) -- A man in civilian clothing opened fire on a base shared with Afghan forces, killing three NATO troops, a spokesman for the International Security Assistance Force said Saturday.

Soldiers detained the gunman shortly after the shooting Friday night, said Maj. Martyn Crighton of the ISAF Joint Command.

ISAF believes the accused gunman was a civilian employee on the base, though the shooting is still under investigation, he said.

The international coalition did not release the nationalities of the soldiers and details about the shooting, including the location.

It was the second shooting Friday targeting NATO troops in Afghanistan.

Three U.S. soldiers were killed when a man in an Afghan military uniform opened fire in the Helmand province, the latest in a series of assaults against NATO troops by Afghans wearing security force uniforms.

The three soldiers were part of a Special Operations mission to stabilize a village in the Sarwan Qala region, a Taliban stronghold, a Defense Department official said on condition of anonymity.

They were meeting with local officials when the shooting occurred, said the official, who was not authorized to release details to the media. NATO and Afghan troops have been searching for the gunman.

"It is too early to say that this latest incident is part of a stepped-up effort on the insurgents' part," White House spokesman Jay Carney said earlier Friday.

"We know they are trying to maximize media exposure of these events but our military believes that the operational impact has been negligible. Nonetheless, these incidents do concern us and our hearts go out to all the victims and their families."

In the strikes, known as "green-on-blue" attacks, Afghan security forces or militants dressed as local police or soldiers target coalition troops.

Earlier this year, Gen. John Allen, commander of all U.S and NATO forces in Afghanistan, said the systems that Afghans and coalition troops had put in place to help prevent these attacks were having an effect.

Allen said coalition officials were working on a new procedure to check the backgrounds of Afghans who sign up for the army or police force, and the Afghans "have taken a lot of steps themselves."

CNN's Chelsea J. Carter, Joe Sterling and Ben Brumfield contributed to this report.