Citing a U.S. military official, CNN reported three or four other troops were wounded in the attack at a base in Helmand province.
The official said attackers used small firearms and rocket-propelled grenades, and may have breached the outer perimeter of the base.
Maj. Martin Crighton, a spokesman for the International Security Assistance Force, said the base took indirect fire, CNN reported.
Crighton said there had been no protests at the base gates of the kind that have sprung up this week at scores of U.S. embassies and consulates, primarily in the Middle East and north Africa.
The base includes Camp Bastion, where Britain's Prince Harry -- who arrived in Afghanistan a week ago to begin a four-month deployment -- was to be based with the 662 Squadron, 3 Regiment Army Air Corps.
Crighton said the prince "was in no way in any danger" from Saturday's attack.
CNN said the attack was over and security patrols were assessing damage.