Four Americans were killed Saturday in a huge explosion inside the largest US military base in Afghanistan, Secretary of Defense Ash Carter said.

The Americans -- two servicemembers and two contractors -- were killed at Bagram Airfield by "an apparent suicide bomber," Carter said in a statement, adding that the explosion wounded 16 other US servicemembers and a Polish soldier participating in US-led NATO military operations.

Carter said he was "deeply saddened" to learn of the news, and expressed his "sincere condolences" to the families of the fallen.

"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," he said.

As for the attackers, "my message is simple. We will not be deterred in our mission to protect our homeland and help Afghanistan secure its own future."

READ: Four killed, 14 wounded in explosion at US base in Afghanistan: NATO

The Taliban claimed responsibility for the morning bombing inside the heavily fortified airfield, located north of the capital Kabul.

The attack highlights rising insecurity in Afghanistan nearly two years after US-led NATO forces formally ended their combat operations.

The United States has around 10,000 troops in Afghanistan, with the largest contingent stationed at the Bagram base.