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An Afghan police officer fatally shot three members of the U.S. Marine Special Operations Forces in southern Afghanistan Friday, according to officials. This is the third Afghan attack on coalition forces this week. A U. S Defense Department official confirmed the deaths Friday but spoke on condition of anonymity because the family notification process was still ongoing.

At least thirty coalition service members have died this year at the hands of Afghan forces or insurgents disguised in Afghan uniforms, according to an Associated Press tally -- a dramatic rise from past years. The assaults have cast a shadow of mistrust over U.S. efforts to train Afghan soldiers and police more than 10 years after the U.S.-led invasion to topple the Taliban's regime for sheltering the al-Qaida leadership.

Friday's shooting took place in Sangin district of Helmand province, said U.S. military spokeswoman Maj. Lori Hodge. She gave no further details and said the military were investigating.

Exactly what happened in the attack was unclear, and there were conflicting accounts. Sangin Gov. Mohammad Sharif said the shooting happened at a police checkpoint after a joint meal and a security meeting, but an Afghan army commander, Farooq Parwani, said that the attack happened on a U.S. base.

Taliban spokesman Qari Yousef Ahmadi said by telephone that the attacker, whom he identified as a member of Helmand police named Asadullah, had joined the insurgency after his attack. Ahmadi said the man had been helping U.S. forces train the Afghan Local Police troops.

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The U.S. is hoping the Afghan Local Police will be a key force to fight the insurgency after most international troops withdraw.

The attack is the latest in a rising number of so-called "green-on-blue" attacks in which Afghan security forces, or insurgents disguised in their uniforms, kill the U.S. or NATO partners who are training Afghans to take over once most international forces leave in 2014. Compared to the 21 attacks this year that killed 30 foreign troops, there were 11 such attacks and 20 deaths in 2011. And in 2007 and 2008, there was a combined total of four attacks and four deaths

On Tuesday, two gunmen wearing Afghan army uniforms killed a U.S. soldier and wounded two others in Paktia province in the east. And on Thursday, two Afghan soldiers tried to gun down a group of NATO troops outside a military base in eastern Afghanistan. No international forces were killed, but one of the attackers was killed as NATO forces shot back.

Also Friday, NATO said another coalition service member died after an insurgent attack in southern Afghanistan. It gave no further details. The death brings to 19 the number of coalition troops killed in Afghanistan this month.

And elsewhere in Helmand province Friday, six Afghan civilians were killed when their car hit a roadside bomb, one of thousands planted by insurgents across the volatile region. Helmand police official Mohammad Ismail Khan said the bomb killed three children, two women and a man.

Meanwhile, the U.S. government identified four Americans killed Wednesday in a twin suicide attack in Afghanistan's east. Secretary of State Hillary Rodham Clinton condemned the bombing by two men wearing suicide vests in the eastern Kunar province. The Taliban also claimed responsibility for that attack.

Clinton's statement said USAID foreign service officer Ragaei Abdelfattah, three coalition service members and an Afghan civilian were killed. A State Department diplomat was injured.

The Defense Department identified the three troops killed in Kunar as Air Force Maj. Walter D. Gray, of Conyers, Ga.; Army Maj. Thomas E. Kennedy, of West Point, N.Y.; and Command Sgt. Maj. Kevin J. Griffin, of Laramie, Wyo.

Associated Press contributed to this report.