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 high casualties were inflicted.
NATO operates with more than 100,000 troops in the country, including some 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.
NATO forces and foreign civilians have also been increasingly attacked by rogue Afghan military and police, eroding trust between the allies.
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, of Mansfield, Georgia, 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.