A man in an Afghan army uniform shot and killed three American service members on Friday morning in southern Afghanistan, the U.S. military command said.
The Taliban claimed the shooter joined the insurgency after the attack.
The shooting happened in Sangin district of Helmand province, said U.S. military spokeswoman Maj. Lori Hodge.
She gave no details and said the military were investigating.
The attack is the third killing this week of coalition soldiers by Afghans they are training to take over responsibility for security once most international forces leave in 2014.
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 was 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.
