A Taliban spokesman sent a text message to journalists Saturday claiming the alleged assailant was an "infiltrator" of the Afghan army.
Attahullah Khogyani, spokesman for the provincial governor in Nangarhar province, said two other United States soldiers are wounded in the attack.
The Taliban claimed responsibility for the attack.
"The Afghan commando was also killed in counterattack".
Meanwhile, NATO forces have declined to comment on the killings.
Taliban spokesman Zabihullah Mujahid said the shooter was a member of the militant group and had killed four Americans and wounded several more. The group is known to inflate the number of its victims.
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In a separate development, eight Taliban militants were killed after an Improvised Explosive Device (IED) they were planting went off prematurely along a road in neighbouring Nehri Sarraj district late Friday. "It seems majority are manipulated and dragged into the trick of "fighting for Islam", he said. This could allow US advisers to work with Afghan troops below the corps level, potentially putting them closer to fighting, a USA official said.
Such strikes have bred deep mistrust between local and foreign forces.
And it's not the first time this year that one has occurred, as NBC News explains: "The attack follows one in March in which three American soldiers were shot and wounded by an Afghan soldier on a base in Helmand province, officials said".
Several members of the Afghan security forces were killed on Friday when a United States military aircraft opened fire during an overnight operation in Helmand province. The U.S. military in a statement apologized for the deaths and said the incident was under investigation.
Gen. John W. Nicholson Jr., the top US military commander in Afghanistan, wants 3,000 to 5,000 more troops to help train Afghanistan's military and police forces as they battle Taliban insurgents, Islamic State militants and other militias, USA officials say. The Afghan special forces units have quickly become the most reliable force in the more than 15-year-old war, American officials believe. US-led North Atlantic Treaty Organisation troops have been at war there since 2001, after the toppling of the Taliban government.
Hundreds of US Marines recently arrived in Helmand to take over an advising mission as part of a NATO-led coalition training and assisting local forces.
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