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.
The shooting occurred late Friday in Afghanistan's volatile Helmand province, the same region where three American soldiers were killed hours earlier by a man in an Afghan military uniform, officials said.
Afghan and ISAF officials offered conflicting accounts over the latest shooting in Garmsir District, where U.S. Marines have been working with Afghan forces at a joint outpost.
An Afghan official said a local police officer opened fire, killing three Marines, while ISAF said the gunman was wearing civilian clothes.
The international coalition did not release the nationalities of the troops or details about the shooting or the location. The gunman was detained by troops shortly after the shooting Friday night, said Maj. Martyn Crighton of the ISAF Joint Command.
Haji Mohammad Khan, the deputy district chief, said a policeman opened fire on the Marines at the outpost located inside the Garmsir police compound.
But Crighton said the shooter was an Afghan civilian working on an installation shared by Afghan and NATO forces. He would not say whether the shooting occurred at the compound in Garmsir.
Brig. Gen. Gunter Katz, an ISAF spokesman, who also described the man as a civilian, said he had been detained after the attack.
It was the second Friday shooting 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.
Afghan President Hamid Karzai condemned the killings of NATO service members and ordered an investigation.
"The enemy who does not want to see Afghanistan have a strong security force, targets military trainers," he said in a statement.
On Friday, White House spokesman Jay Carney said it was "too early to say" that the latest incident was part of a stepped-up effort by insurgents.
"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.
Asked about the incidence of such attacks, Katz told reporters in Kabul that ISAF's assessments indicated "the infiltration rate is very, very low," but that more could be done to educate both sides and build mutual respect.
"If you just look at the statistics, what I can see compared to last year is that we have much more soldiers in the field," he said. "We have almost 500,000 soldiers fighting together right now, and in the years before, those numbers were much smaller."
Afghan security forces have also been targeted by their countrymen.
On Saturday, an Afghan policeman killed 10 fellow police officers in the Dilaram district of the southern Nimruz province, said Omar Baluch, a spokesman for the provincial governor.
The shooter, who is believed to have links to the Taliban, was killed by other policemen, said Baluch. An investigation into the incident continues, he added.
Earlier this year, Gen. John Allen, commander of all U.S and NATO forces in Afghanistan, said systems put in place by Afghans and coalition troops to prevent such 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."
Popalzai reported from Kabul, Carter from Atlanta. CNN's Joe Sterling, Laura Smith-Spark and Ben Brumfield contributed to this report.