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.
"Now, he is with us," Ahmadi said.
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
The NATO coalition says it takes such attacks seriously, but it insist they are not a sign of trouble for the plan to hand over security to Afghan forces.
"We are confident that those isolated incidents will have no effect on transition or on the quality of our forces," said Brig. Gen. Gunter Katz, a spokesman for NATO troops.
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.