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U.S. forces dropped the largest non-nuclear bomb ever used in a strike against ISIS in eastern Afghanistan on Thursday, according to Pentagon officials, reportedly killing 36 militants.

The U.S. dropped a GBU-43 bomb, nicknamed the "mother of all bombs," on ISIS fighters and tunnels and caves used by the terror group in the country’s Nangarhar province, officials said. It was dropped from an aircraft.

Afghanistan's defense ministry said the attack left 36 ISIS fighters dead — a claim the ISIS-affiliated news agency Amaq later denied.

Gen. John W Nicholson, commander of U.S. forces in Afghanistan, said U.S. forces in the area had seen no evidence nor heard any reports of civilian casualties.

"The weapon used ... is designed to destroy caves and tunnels. This was the right weapon against the right target," he told reporters Friday. "I want to reassure the people of Afghanistan that we took every possible precaution to prevent civilian casualties."

He added: "We are united with the Afghan government to prevent terrorists from establishing safe havens in Afghanistan."

Late Thursday, President Donald Trump told reporters at the White House that he was "so proud of our military."

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"Another successful job. We're very, very proud of our military," he said.

The president did not answer directly when asked by a reporter if he had authorized the use of the massive bomb.

"Everybody knows exactly what happened," Trump said. "We have the greatest military in the world...We have given them total authorization...If you look at what's happened over the last eight weeks and compare that really with what's happened over the last eight years, you'll see there's a tremendous difference. Tremendous difference."

White House Press Secretary Sean Spicer said the GBU-43 bomb was used Wednesday around 7 p.m. local time in Afghanistan. He called the weapon "large, powerful" and accurate.

"The U.S. takes the fight against ISIS very seriously," Spicer said during the daily press briefing.

The press secretary did not answer reporters' questions on what led to the strike — a stark contrast to how the administration outlined what led to a missile attack in Syria the week before.

Related: How Big is the GBU-43?

Spicer referred questions to the Department of Defense, declining to "get into details" on whether Trump had ordered the bombing. President authorization is not required for use of the GBU-43.

Military experts said the bomb's massive blast is terrifying and may have been intended to send a message to ISIS in Afghanistan.

U.S. soldiers take up positions during an operation against ISIS militants in Afghanistan's Nangarhar province on April 11. Noorulah Shirzada / AFP - Getty Images file

Former Afghanistan President Hamid Karzai strongly condemned the use of the bomb on Twitter.

I vehemently and in strongest words condemn the dropping of the latest weapon, the largest non-nuclear #bomb, on Afghanistan by US...1/2 — Hamid Karzai (@KarzaiH) April 13, 2017

Hans Kristensen, director of the nuclear information project at the Federation of American Scientists, said that 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 GBU-43.

The GBU-43/B Massive Ordnance Air Blast bomb in an image provided by Eglin Air Force Base. Eglin Air Force Base via AP

The GBU-43 is 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 on whether to build miniature nuclear weapons.

"We have people arguing for new min-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 (GBU-43) 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?"