An American soldier was stabbed to death by an Afghan teenager as he played with a group of local children.
The killing comes as the monthly US 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.
Sargeant Michael Cable, 26, was guarding officials meeting in a province near the border with Pakistan when he was attacked last Wednesday.
It came after the soldiers had secured the area for the meeting, but one US official said the killer 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.
They said the attacker was thought to be about 16 years old.
The Afghan and American dignitaries were attending the swearing-in ceremony of Afghan Local Police in Shinwar district in Nangarhar province. 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.
The attacker is thought to have fled to nearby Pakistan.
The Pentagon said last week that sergeant Cable died from injuries sustained when his unit was attacked by enemy forces.