istan -- Two American soldiers were killed Thursday by a civilian employee of the Afghan army, according to Afghan officials -- the latest in a concerted series of attacks in which Afghan soldiers, police or government workers have turned their guns on Western allies.

REPORTING FROM KABUL, Afghan

The NATO force confirmed the shooting deaths of two coalition soldiers, saying the attack was carried out by a man believed to be an Afghan soldier and a second Afghan in civilian clothing.

The escalating attacks -- so-called green-on-blue shootings -- have called into question whether an ambitious NATO effort to train Afghan forces to take over responsibility for safeguarding the country can move forward. Handing over security responsibility to the Afghan police and army is central to NATO’s exit strategy.

Four Americans died in two such assaults last week -- two troops shot by an Afghan soldier at a U.S.-run base in eastern Afghanistan while riots raged outside, and two military officers shot execution-style inside a tightly guarded government ministry in the capital, Kabul.

Niaz Mohammad Sarhadi, the chief of Kandahar’s Zhari district, said the attack took place before dawn at a joint coalition-Afghan outpost in the village of Sangisar. He identified the assailant as a civilian working for the Afghan military as a literacy tutor, and said the shooter had snatched a weapon from an Afghan soldier.

He said that in addition to the two Americans killed, a third was wounded. The assailant was killed along with an Afghan soldier, and another Afghan soldier was injured, Sarhadi said.

The Western military account differed somewhat, describing two assailants having “turned their weapons indiscriminately” against NATO troops and other Afghan soldiers who were present. It did not identify the nationalities of those killed and wounded.

Dozens of such rogue shootings have taken place in the last two years, but the pace has intensified sharply in recent months. The Kandahar attack also comes at a time of heightened tensions over deadly riots that broke out last week over the burning of copies of the Koran -- the Muslim holy book -- by U.S. personnel at a base north of Kabul. More than 30 people died in days of violent protests.

American officials said the religious texts were sent to the “burn pit” at Bagram air base by accident, and the top U.S. commander in Afghanistan and President Obama, as well as other senior U.S. officials, have apologized. Parallel U.S and Afghan investigations are underway to determine how the decision to destroy the volumes in the base incinerator was made.

Devout Muslims hold the Koran to be the literal word of God, and desecrating the book is considered an act of extreme sacrilege.

U.S. military officials have said Afghan detainees at a high-security prison adjacent to the base were using the religious texts to pass “subversive” handwritten messages back and forth. Afghan officials have demanded that those responsible for the Koran burning be put on trial.

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-- Laura King