A United States military statement said the soldier was killed when a joint patrol of Afghan and American troops clashed with a force of between 10 and 15 fighters near Deh Rawood in Uruzgan province on Thursday.
The statement, issued from the Bagram air base on Friday, said an American A-10 Thunderbolt aircraft and AH-64 Apache helicopter opened fire on the fighters, killing one of them.
An American special forces soldier and an Afghan soldier were also injured in the clash.
The death brings to 36 the number of US troops killed in military operations in Afghanistan since the launch of Operation Enduring Freedom in October 2001.
Two US agents of the Central Intelligence Agency (CIA) were killed last Saturday during a hunt for al-Qaida and Taliban fighters on the Afghan-Pakistan border.
US nationals William Carlson, 43, and Christopher Glenn Mueller, 32, former military special operations officers, were killed in an ambush near the southeast town of Shkin on the Pakistani border.
Some 12,500 US-led coalition troops are facing an increased threat from regrouping Taliban members as they hunt down members of the former Afghanistan government and their al-Qaida allies, mostly along the rugged and porous Afghan-Pakistan border.