(NBC News) - U.S. forces dropped one of the biggest non-nuclear weapons in its arsenal in a large-scale strike against ISIS in eastern Afghanistan on Thursday, according to Pentagon officials.

The U.S. dropped a GBU-43 bomb, nicknamed "the mother of all bombs", on ISIS fighters and facilities in the country's Nangarhar province, officials said.

Military officials believe it is the first time the weapon was used on the battlefield.

White House Press Secretary Sean Spicer said that the GBU 43 weapon was used Wednesday around 7 p.m. local time in Afghanistan. He called the weapon "large, powerful" and accurate. The strike targeted tunnels and caves used by ISIS.

"The U.S. takes the fight against ISIS very seriously," Spicer said during the daily briefing, calling the military move a part of that effort.

The White House did not answer reporters' specific questions on what led to the strike —a stark contrast to how the administration outlined what led to a military strike in Syria the week before.

Spicer referred questions to the Department of Defense, declining to "get into details" on whether Trump ordered the attack.

Military officials did not immediately know how many ISIS fighters were killed or if any civilians died in the attack.

"As ISIS-K's losses have mounted, they are using IEDs, bunkers and tunnels to thicken their defense," Gen. John W. Nicholson, the commander of U.S. forces in Afghanistan, said in a statement. "This is the right munition to reduce these obstacles and maintain the momentum of our offensive against ISIS-K."

Hans Kristensen, director of the nuclear information project at the Federation of American Scientists, said the explosive power of even the smallest nuclear weapon in the U.S. arsenal, the B-61 bomb, is "an order of magnitude" larger than the GB-43, the largest non-nuclear bomb the U.S. has available.

The B-61 bomb, which is only deployed in Europe, is a tactical weapon that can be used to destroy city centers or large scale troop concentrations in a ground war.

The GBU-43 is also about half the size of the smallest nuclear weapon ever built, the Davy Crockett nuclear artillery shell, retired in the late 1960's.

Kristensen said there is a debate inside the defense community now on whether to build miniature nuclear weapons.

"We have people arguing for new mini-nukes," he said. "Here you have a case where the U.S. felt all it needed was a conventional whopper."

"The big unknown with this bomb is can you get the detonation point close enough to what is in the tunnel," he said. "How deep does it go in? Does this just destroy the entrance?"

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