A suicide bomber killed at least 12 American servicemembers in an attack in the Afghan capital of Kabul, news reports said.
More than 20 people in all were killed in the attack on an armored bus that plied a route popular with foreign military trainers.
The New York Times reported it was the worst loss of life among American troops in the capital in several years.
At least eight civilians were among the dead, including children and Afghan police officers.
A Taliban spokesman claimed credit for the attack.