KABUL, Afghanistan (AP) – An explosion at a U.S. airfield in Afghanistan early Saturday morning killed four people, the U.S. military said.

U.S. Secretary of Defense Ash Carter said in a statement that two U.S. servicemen and two American contractors were killed in the explosion. Another 16 U.S. service members and one Polish soldier were wounded.

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“For those who carried out this attack, my message is simple: We will not be deterred in our mission to protect our homeland and help Afghanistan secure its own future,” Carter said.

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The Taliban claimed responsibility for the attack, which they said was carried out by a suicide bomber inside the base. Zabihullah Mujahid, a spokesman for the insurgent group, said the attack had been planned for four months.

The Taliban regularly fire rockets at Bagram from outside its perimeter.

Abdul Wahid Sediqqi, spokesman for the governor of Parwan province, where the air field is based, told The Associated Press that laborers employed at the base usually line up at the gates around dawn and the attacker could have been among the men entering the base Saturday.

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(Copyright 2016 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)