The Pentagon says a suicide bomber attacked NATO's Bagram airfield in Afghanistan, killing four Americans and wounding many others.
The Defense Department said in a statement that the blast on November 12 killed two U.S. service members and two U.S. contractors, and 16 other U.S. service members were wounded.
A Polish soldier was also wounded.
Secretary of State John Kerry condemned the attack on the U.S. military's largest base in Afghanistan, calling it "abhorrent and cowardly."
He pledged to reporters while in New Zealand on November 13 that the U.S. mission in Afghanistan "will not be deterred by these individual acts."
The Defense Department gave few details about the location and circumstances of the blast.
A statement released by the NATO-led Resolute Support mission said the explosion occurred at the base north of Kabul at dawn on November 12, and that force-protection and medical teams were at the scene.
Local media reported that a man posing as a laborer got access to the base and detonated a suicide vest.
Taliban spokesman Zabihullah Mujahid posted a statement on Twitter claiming responsibility for the attack.
Bagram has frequently come under attack by Taliban insurgents.
In December, a motorcycle-riding Taliban suicide bomber killed six U.S. soldiers near the base.
It was one of the deadliest attacks on foreign troops in the country in 2015.
Late on November 10, the Taliban carried out a suicide attack in the northern city of Mazar-e Sharif that killed four people and wounded more than 100.
About 10,000 U.S. troops are stationed at Bagram on a mission to support and train Afghan security forces.
With reporting by Reuters, dpa, and AFP