A U.S. official said early indications were that the attacker did not breach the base's perimeter. The official spoke on condition of anonymity because the matter was in the early stages of investigation.
Earlier, Ahmad Javed Faisal, spokesman for the governor of Kandahar province, said the attack occurred after dusk and was carried out by a bomber on a motorcycle. There often are conflicting reports in the immediate aftermath of attacks.
Associated Press writers Robert Burns and Amir Shah contributed to this report from Kabul.