Taliban insurgents killed eight civilians earlier this week when a remote-control bomb was Musadeq Sade/Associated Press (detonated aboard a bus near Kabul, the capital.)

A man in an Afghan army uniform shot and killed three American service members Friday morning in southern Afghanistan, the third attack on coalition forces by their Afghan counterparts in a week.

The Taliban claimed the shooter joined the insurgency after the attack.

The shooting took place in Sangin district of Helmand province, U.S. military spokeswoman Maj. Lori Hodge told reporters. She gave no details.

Taliban spokesman Qari 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 Afghan local police. The U.S. hopes the police will be a key force to fight the insurgency after most international troops withdraw.

The attack is the third this week on coalition soldiers by Afghans they are training to take over once most international forces leave in 2014.

On Tuesday, two gunmen wearing Afghan army uniforms killed an American 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 returned fire.

This year has seen a rising number of so-called "green-on-blue" attacks in which Afghan security forces, or insurgents disguised in their uniforms, kill their American or NATO partners.

So far this year, 30 coalition troops have been killed in 20 such attacks, according to an Associated Press tally. That compares with 11 fatal attacks and 20 deaths the previous year. In 2007 and 2008 there was a combined total of four attacks and four deaths.

The U.S. government also identified four Americans killed earlier in the week in a twin suicide attack in Afghanistan's east. Secretary of State Hillary Rodham Clinton condemned Wednesday's attack by two men wearing suicide vests in the eastern Kunar province.

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 wounded.

The U.S. Defence 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.

The Taliban have not limited their recent attacks to NATO troops. On Tuesday, the the insurgents detonated a remote-controlled bomb aboard a bus, killing eight civilians.