US forces in Afghanistan on Thursday struck a militant Islamic State (IS) tunnel complex in eastern Afghanistan with “the mother of all bombs,” the largest non-nuclear weapon ever used in combat by the US military, Pentagon officials said.

The bomb, known officially as a GBU-43B, or massive ordnance air blast weapon, unleashes 11 tons of explosives.

When it was developed in the early 2000s, the Pentagon did a formal review of legal justification for its combat use.

The US military headquarters in Kabul said in a statement that the bomb was dropped at 7:32 p.m. local time Thursday on a tunnel complex in Achin district of Nangarhar province, where the Afghan affiliate of the IS group has been operating.

The target was close to the Pakistani border.

The US estimates 600 to 800 IS fighters are present in Afghanistan, mostly in Nangarhar. The US has concentrated heavily on combatting them while also supporting Afghan forces battling the Taliban.

Just last week a US Army Special Forces soldier, Staff Sgt. Mark R. De Alencar, 37, of Edgewood, Maryland, was killed in action in Nangarhar.

In its 2003 review of the legality of using the bomb, the Pentagon concluded that it could not be called an indiscriminate killer under the Law of Armed Conflict.

“Although the MOAB weapon leaves a large footprint, it is discriminate and requires a deliberate launching toward the target,” the review said, using the acronym for the bomb.

Adam Stump, a Pentagon spokesman, said the bomb was dropped from a U.S. MC-130 special operations transport. He said the bomb had been brought to Afghanistan “some time ago” for potential use.

Army Gen. John W. Nicholson, commander of U.S. forces in Afghanistan, said in a written statement that the strike was designed to minimise the risk to Afghan and US forces conducting clearing operations in the Achin area “while maximising the destruction” of IS fighters and facilities.

He said IS has been using improvised explosive devices, bunkers and tunnels to strengthen its defenses.

“This is the right munition to reduce these obstacles and maintain the momentum of our offensive against ISIS-K,” he added, using the US military's acronym for the IS affiliate.

White House spokesman Sean Spicer said IS fighters had used the tunnels and caves in Achin to maneuver freely.

“The United States takes the fight against ISIS very seriously and in order to defeat the group we must deny them operational space, which we did,” Spicer said.