KABUL, Afghanistan - A vehicle driven by a suicide bomber exploded at the gate of a major U.S. military base in eastern Afghanistan on Wednesday, killing the attacker and three Afghans, Afghan police said. The Taliban claimed responsibility for the attack.

Police Gen. Abdul Qayum Baqizai said a local guard who questioned the vehicle driver at the gate of Camp Chapman was killed along with two civilians and the assailant. The camp is located adjacent to the airport of the capital of Khost province, which borders Pakistan.

Chapman and nearby Camp Salerno had been frequently targeted by militants in the past, but violent incidents have decreased considerably in recent months.

Taliban spokesman Zabihullah Mujahid said in an e-mail that the bomber targeted Afghan police manning the gate and Afghans working for the Americans entering the base. He claimed that high casualties were inflicted.

NATO operates with more than 100,000 troops in the country, including 66,000 American forces. It is handing most combat operations over to the Afghans in preparation for a pullout from Afghanistan in 2014. Militant groups, including the Taliban, rarely face NATO troops head-on and rely mainly on roadside bombs and suicide attacks.

On Tuesday, the Interior Ministry said a policewoman who killed an American contractor in Kabul a day earlier was a native Iranian who came to Afghanistan and displayed "unstable behavior" but had no known links to militants.

The policewoman, identified as Sgt. Nargas, shot Joseph Griffin, 49, of Mansfield, Ga., on Monday, in the first such shooting by a woman in the spate of insider attacks. Nargas walked into a heavily guarded compound in the heart of Kabul, confronted Griffin, and shot him once with her pistol.

The U.S.-based security firm DynCorp International said on its website that Griffin was a U.S. military veteran who was under contract to the NATO military command to advise the Afghan police force.