The insurgent Taliban movement claimed responsibility for the attack near the eastern town of Khost, which was hit by several suicide blasts on Tuesday that left up to nine Afghans dead.
"Twenty-one local nationals were wounded and seven were killed," Specialist April Campbell told AFP from the main US base in Afghanistan at Bagram, north of the capital Kabul.
The bomb exploded at a gate where labourers were waiting to enter the base to start work, an AFP reporter in the town said. Most of the casualties were likely to be these men, but this could not immediately be confirmed.
The provincial police chief Abdul Qayoum Baqizoi said: "It was a car bomb but we have no information about the casualties at this point."
A man who said he was a local member of the Taliban told reporters that the attacker was one of 30 whom the insurgent movement said Tuesday had entered the town to carry out a wave of attacks.
The militant, who gave his name as Salahuddin, warned there would be more strikes on the city.
Eleven suicide bombers were killed in Khost on Tuesday in a nearly day-long standoff that Afghan officials said killed between six and nine Afghans and wounded around 30.
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