The attackers had detonated a car bomb at an entrance to Camp Chapman, a secretive facility manned by US forces and private military contractors, said Mubarez Mohammad Zadran, a spokesman for the provincial governor.
But he had little immediate information on any damage or casualties.
"I am aware of a car bomb attack at one of the gates in the US base, but we are not allowed there to get more details," the spokesman said.
The attack came just three days after more than 140 Afghan soldiers were killed in an attack on their base by Taliban fighters disguised in military uniforms.
(Reporting by Ahmad Shah and Josh Smith, Writing by James Mackenzie, Editing by Clarence Fernandez)