Three American troops and an Afghan interpreter were killed Tuesday when a suicide bomber rode a donkey into a joint Afghan-NATO military convoy and blew himself up, according to a report from Agence France-Presse.
The attack brings the number of U.S. casualties to 2,064 according to the website iCasualties.org.
U.S. military forces are scheduled to withdraw from Afghanistan by the end of 2014, and Barack Obama's administration has considered speeding up the timeline as relations between the U.S. and Afghanistan have deteriorated in recent months.
NATO's International Security Assistance Force confirmed three casualties in a bombing in eastern Afghanistan but gave no details.
Afghan authorities said the attack happened in Wardak province, a hotbed of the Taliban insurgency just south of the capital Kabul.
The Taliban has claimed responsibility for the attack.
Four Afghan soldiers who were also in the convoy in Sayedabad district were wounded.
Along with roadside bombings, suicide attacks — often carried out by multiple insurgents storming buildings and government facilities in big cities — are the group's main tactic.
Wardak is traversed by the highway between Kabul and Afghanistan's southern and western provinces, including the key city of Kandahar in the south.
There have been growing concerns that Afghanistan could descend into civil war without the support of U.S. forces. A small residual group of American soldiers will remain in Afghanistan in a support role. The administration hasn't yet set a date for the end of the support mission.
Al Jazeera and wire services