A U.S. service member was killed Thursday in Afghanistan’s eastern Paktia province when a man in an Afghan National Army uniform opened fire at a joint military training base, provincial officials said.

The assailant was killed in return fire by NATO forces, said Rohullah Samoon, a spokesman for Paktia’s governor.

“It’s too early to say whether the Afghan soldier was a Taliban infiltrator,” Samoon said, adding that a joint investigation by Afghan and coalition forces was underway into the incident in Paktia’s Gerda Seri district.

Taliban spokesman Zabiullah Mujahid said in a Twitter message that his group was responsible for the attack on U.S. soldiers. NATO confirmed the death but didn’t immediately provide the victim’s nationality, in keeping with its policy.

This was the second so-called insider attack in Paktia in less than a week. Three U.S. troops were killed in a similar incident Saturday.

Insider attacks, involving gunmen wearing Afghan army or police uniforms, along with low-cost deadly roadside bombs, have proved effective for the Taliban and other insurgent groups against sophisticated modern militaries for several reasons, analysts said.

They undermine morale by sowing distrust between foreign and Afghan forces at a time when international troops are handing over more responsibility for security to local personnel, they said.

They also said such assaults feed a line of propaganda that “real” patriot Afghans oppose foreign forces in their country, and put pressure on foreign combat troops — who are scheduled to leave by the end of 2014 — to speed up their departure.

Insider attacks killed 62 people in 47 incidents in 2012, according to NATO, compared with 35 killed in 21 attacks in 2011.

Before Thursday’s attack, the number of foreign military deaths in Afghanistan since 2001 stood at 3,381, including 2,278 Americans, according to iCasualties.org, an independent website. So far in 2013, 132 coalition troops have died, including 104 Americans, a decline as Afghan troops take more of a lead role.