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 . Official : Man in Afghan security uniform kills 3 U.S. troops . 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 .
An Afghan policeman kills 10 fellow officers , an official in Nimruz province says . The Taliban claimed responsibility for the attack on a police station and said it was revenge for recent U.S. airstrikes against their fighters. The shooting occurs at a base shared by Afghan and NATO troops , the official says . Earlier in the day , a man in an Afghan security force uniform kills three U.S. soldiers .