KABUL, Afghanistan — A man in an Afghan uniform shot and killed three American troops Friday morning in southern Afghanistan, the U.S. military command said, in the third attack on coalition forces by their Afghan counterparts in a week. The Taliban claimed the shooter joined the insurgency after the attack.

So far this year, at least 21 similar attacks — in which Afghan forces or insurgents disguised in Afghan uniforms have turned their guns on international troops — have killed 30 coalition service members, according to an Associated Press tally.

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.

The U.S. military said it was also not clear whether the attacker wore an Afghan army or police uniform.

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.

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.