A suicide bomber who apparently infiltrated Bagram Air Base on Saturday in Afghanistan killed two US soldiers and two US contractors, Defence Secretary Ash Carter said.
The blast wounded 16 US service members and one Polish soldier participating in NATO's Resolute Support mission in Afghanistan, he said.
Bagram in Parwan province in central Afghanistan is the largest US military airbase in the country.
"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.
The suicide bomber blew himself up near a dining hall inside the base, Bagram district chief Haji Abdul Shukur Quddusi said.
Carter vowed to improve security at Bagram: "Force protection is always a top priority for us in Afghanistan, and we will investigate this tragedy to determine any steps we can take to improve it."
Taliban spokesman Zabihullah Mujahid said the group was responsible for the "suicide attack" and that 67 US soldiers had been killed and wounded. The guerilla group is known for making exaggerated claims.
"Martyrdom seeker Mujahid - Hafiz Muhammad Parwani - had already managed to enter the airfield very tactically along with necessary toll and explosives," Mujahid said, claiming that the attack was four months in the planning.
On Thursday, four people were killed and 128 wounded when a suicide bomber drove an explosives-laden lorry into the German consulate in Mazar-e Sharif in northern Afghanistan.
Bagram is one of the most secure US military bases in Afghanistan with multiple perimeters, guard towers and a blimp watching the camp 24 hours a day.
If confirmed, the attack would be the first infiltration since the airfield was taken over by US forces after the former Taliban regime in Kabul was toppled in late 2001.
Although attacks and bombings have taken place on the outer security walls and rockets have been fired into the base, an attack has never taken place inside the heavily guarded camp.
In December, six US soldiers were killed while on patrol 10 kilometres outside Bagram by a suicide bomber on a motorbike.
Nearly 10,000 US troops are currently stationed in Afghanistan assisting Afghan security forces under the Train, Advise and Assist (TAA) mission.
By Mohammad Jawad and Abdul Qadir Sediqi
