The killing comes as the monthly US death toll rose sharply during March to 14, with the start of the spring fighting season, when the Taleban and other insurgents take advantage of improved weather to step up attacks.
Sergeant Michael Cable, 26, was guarding Afghan and American officials meeting in a province near the border with Pakistan when the stabbing occurred last Wednesday, two senior officials said yesterday.
The attack occurred after the soldiers had secured the area for the meeting, but one of the US 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 aged about 16, but this could not be verified.
The Afghan and American dignitaries were attending the swearing-in ceremony of the Afghan Local Police in Shinwar district in Nangarhar province, said senior district official Zalmai Khan.
Afghan Local Police, or ALP, recruits are drawn from villages and backed by the US 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, Mr Khan said, adding that the young man had escaped to nearby Pakistan.
Taleban spokesman Zabiullah Mujahid said the young man had been 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 Sgt Cable, of Philpot, Kentucky, died from injuries sustained when his unit was attacked by enemy forces.