A man wearing an Afghan army uniform shot dead a U.S. service member in the east of the country, one of two NATO troops killed on Friday, military officials said. The Taliban took credit for the attack.

Also Friday, the U.S.-led military coalition offered condolences to the families of Afghan civilians who were killed in airstrikes earlier this month.

The shooting was the 15th incident this year in which Afghan soldiers or insurgents disguised in military uniforms have turned their weapons on foreign troops. The killings have increased the level of mistrust between the U.S.-led coalition and its Afghan partners and raised questions about the readiness of local forces to take over from NATO ahead of a 2014 deadline for the withdrawal of foreign combat troops.

An Afghan defense official said the incident took place in Kunar province in northeast Afghanistan. The official spoke on condition of anonymity because he was not authorized to brief the media.

NATO did not disclose the nationality of the trooper killed, but a Pentagon spokesman, Navy Capt. John Kirby, said the service member was an American.

Kirby said U.S. and allied officials are troubled that these attacks are continuing with some regularity despite efforts to improve the vetting of Afghans who are recruited into the army and police.

"It continues to be a very worrisome issue for us and for our Afghan partners," he said.

The coalition said an investigation into Friday's attack was under way.

In a statement, Taliban spokesman Zabiullah Mujahid claimed the insurgent group was behind the shooting. The Taliban regularly take credit for attacks in the country, even if they were not involved.

The threat of Afghan soldiers or militants disguised in uniforms turning their guns on NATO troops has long existed but has grown more deadly over the past five years. While there were only a few deaths reported in 2007 and 2008, 35 foreign troops were killed in such attacks last year.

So far this year, there have been 15 such attacks, killing 20 NATO service members, according to Lt. Col. Jimmie Cummings, a spokesman for the U.S.-led NATO coalition.

The coalition routinely reports each time an American or other foreign soldier is killed by an Afghan in uniform, but the military is underreporting the number of overall attacks. The Associated Press reported last month that the coalition does not report attacks in which an Afghan wounds — or misses — his U.S. or allied target. It also doesn't report the wounding of troops who were attacked alongside those who were killed.

U.S. officials say that in most cases the rogue soldiers are motivated not by sympathy for the Taliban or on orders from the insurgents, but rather act as a result of personal grievances against the coalition.

NATO said a second service member died following an insurgent attack in southern Afghanistan, but did not provide further details about the death.

The coalition offered condolences to the families of Afghan civilians killed earlier this month in airstrikes in Helmand and Badghis provinces.

"Unfortunately, the preliminary investigations into these events have determined that our actions have resulted in a number of deaths and injuries to Afghan civilians," the coalition said in a statement late Friday.

U.S. Marine Gen. John Allen, the top commander of U.S. and NATO forces in Afghanistan, will brief Afghan President Hamid Karzai on the initial investigations into the civilian deaths, the coalition said.

The Taliban kill more civilians than foreign forces, but the deaths of citizens caught in the crossfire of the decade-long war continue to be an irritant in Karzai's relationship with his international partners. Earlier this week, the Afghan president warned that civilian casualties caused by NATO airstrikes could undermine the strategic partnership agreement he just signed with the U.S. The document governs the relationship between the two countries from the end of 2014 until 2024.

The coalition did not disclose the number of civilians killed in the two airstrikes on May 4 and 6 in Helmand and Badghis provinces, respectively.

Abdul Ghani Sabary, the deputy governor of Badghis, said the strike in Bala Murghab district killed eight civilians and wounded 10 others.

Gulab Mangal, governor of Helmand province, said six civilians died in the airstrike in Sangin district.

Afghan officials said four other civilians were killed this month in airstrikes in Logar and Kapisa provinces.


Associated Press writers Chris Blake in Kabul and Robert Burns in Washington contributed to this report.