The killing comes as the monthly U.S. death toll rose sharply in March to 14 with the start of the spring fighting season when the Taliban and other insurgents take advantage of improved weather to step up attacks.
Sgt. Michael Cable, 26, was guarding Afghan and U.S. officials meeting in a province near the border with Pakistan when the stabbing occurred last Wednesday, two senior U.S. officials said. They spoke on condition of anonymity because the investigation is ongoing.
The attack occurred after the soldiers had secured the area for the meeting, but one of the U.S. officials said the youth was not believed to have been a member of the Afghan security forces or in uniform so it was not being classified as an insider attack.
The official said the attacker was thought to be about 16 years old, but the age couldn't be verified.
The Afghan and American dignitaries were attending the swearing-in ceremony of Afghan Local Police in Shinwar district in Nangarhar province, senior district official Zalmai Khan said. Afghan Local Police, or ALP, recruits are drawn from villages and backed by the U.S. military.
The soldier was playing with a group of children outside when the attacker came from behind and stabbed him in the neck with a large knife, Khan said, adding the young man had escaped to nearby Pakistan.
Taliban spokesman Zabiullah Mujahid said the young man was acting independently when he killed the soldier but had joined the Islamic militant movement since fleeing the scene.
The Pentagon said in a statement last week that Cable, of Philpot, Ky., died from injuries sustained when his unit was attacked by enemy forces.
At least 14 U.S. soldiers died in March, compared with four in the previous two months, according to an Associated Press tally.
The number of American troops killed in Afghanistan has dropped sharply as international forces increasingly take a back seat while preparing to end their combat mission by the end of 2014. But they continue to face dangers ranging from roadside bombs to attacks by their Afghan counterparts or insurgents disguised as government forces.