KABUL, Afghanistan — A man in an Afghan uniform shot and killed three American troops today 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.

Today’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.