Kabul (AFP) - Four Americans were killed on Saturday in a suicide bombing inside the largest US military base in Afghanistan, Pentagon chief Ashton Carter said, in a major breach of security.
The Taliban claimed responsibility for the bombing inside the heavily fortified Bagram Airfield, north of the capital Kabul, which left 16 other US service members and a Polish soldier wounded as the insurgents ramp up attacks on Western targets.
The dead included two American soldiers and two contractors, in an assault which highlights rising insecurity in Afghanistan nearly two years after US-led NATO forces formally ended their combat operations.
"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," defense secretary Carter said, adding he was "deeply saddened" by the news.
"We will not be deterred in our mission to protect our homeland and help Afghanistan secure its own future."
The blast was caused by a suicide attacker who blew himself up near a dining facility inside the base, said Waheed Sediqi, spokesman for the governor of Parwan province where Bagram is located.
"The attacker was one of the Afghan labourers working there," Sediqi told AFP.
Bagram district governor Abdul Shakoor Quddusi described the explosion around 0100 GMT as "powerful", saying it reverberated across the area. The base was put on lockdown following the attack.
The attack represents a brazen security breach inside one of the most heavily guarded military installations in Afghanistan.
The United States has around 10,000 troops in Afghanistan, with the largest contingent stationed at the Bagram base.
The base is enveloped by multiple layers of Afghan and American guards, security cameras and watch towers, and a surveillance blimp keeps a close watch on the surrounding areas.
- Rising insecurity -
"To the family and friends of those who lost their lives today, we share your loss and our thoughts are with you. We offer you our deepest condolences," said John Nicholson, the top US and NATO commander in Afghanistan.
"For the family and friends of those wounded in today's attack, let me assure you they are receiving the best care possible, and we will keep them in our thoughts today."
Taliban spokesman Zabihullah Mujahid said one of the insurgent group's suicide bombers carried out the assault, claiming it inflicted "heavy casualties on US invaders".
The militants are stepping up attacks nationwide before the onset of winter, when the fighting usually ebbs.
Bagram Airfield has frequently come under attack by Taliban insurgents.
Last December, a motorcycle-riding Taliban suicide bomber killed six US soldiers near the base in one of the deadliest attacks on foreign troops in the country in 2015.
The latest assault came after a powerful Taliban truck bomb struck the German consulate in Afghanistan's northern Mazar-i-Sharif city late Thursday, killing at least six people and wounding more than 100 others.
The uptick in attacks comes just days after a bitter US presidential election.
Afghanistan got scarcely a passing mention in the election campaign -- even though the situation there will be an urgent matter for the new president.
President-elect Donald Trump is set to inherit America's longest war with no end in sight.