The United States dropped a massive GBU-43 bomb, nicknamed the "mother of all bombs," in eastern Afghanistan on Thursday against a series of caves used by ISIS militants, the military said.

The 9.8-tonne GBU-43 bomb was dropped from an MC-130 aircraft in the Achin district of Nangarhar province, close to the border with Pakistan, Pentagon spokesperson Adam Stump said.

It was the first time the United States has used this size of conventional bomb in a conflict.

U.S. President Donald Trump described the bombing as a "very successful mission." It wasn't immediately clear how much damage the device did.

The bomb was first tested in March 2003, just days before the start of the Iraq war. When it was developed in the early 2000s, the Pentagon did a formal review of legal justification for its combat use.

During last year's presidential election campaign, Trump vowed to give priority to destroying ISIS. Last week, he ordered a cruise missile attack on a Syrian government airbase in retaliation for a poison gas attack.

"If you look at what's happened over the last eight weeks and compare that really to what's happened over the last eight years, you'll see that there's a tremendous difference," Trump told reporters at the White House on Thursday.

'Large, powerful, accurately delivered'

The security situation remains precarious in Afghanistan, with a number of militant groups trying to claim territory more than 15 years after the U.S. invasion that toppled the Taliban government. Last week, a U.S. soldier conducting operations against ISIS was killed in the district where the bomb was later dropped. So far, Trump has offered little clarity about a broader strategy for Afghanistan, where some 8,400 U.S. troops remain. White House spokesperson Sean Spicer opened his daily news briefing speaking about the use of the bomb.

He said the bombing "targeted a system of tunnels and caves that ISIS fighters used to move around freely, making it easier for them to target U.S. military advisers and Afghan forces in the area."

Spicer said the bomb was dropped around 7 p.m. local time and described it as "a large, powerful and accurately delivered weapon." U.S. forces took "all precautions necessary to prevent civilian casualties and collateral damage," he said.

'Maximizing destruction'

Army Gen. John W. Nicholson, commander of U.S. forces in Afghanistan, said in a news release the strike was designed to minimize the risk to Afghan and U.S. forces conducting clearing operations in the Achin area "while maximizing the destruction" of ISIS fighters and facilities.

.<a href="https://twitter.com/USFOR_A">@USFOR_A</a> <a href="https://twitter.com/hashtag/US?src=hash">#US</a> Forces targets ISIS-K stronghold, drops GBU-43 <a href="https://twitter.com/hashtag/MOAB?src=hash">#MOAB</a> bomb on <a href="https://twitter.com/hashtag/ISIS?src=hash">#ISIS</a> <a href="https://t.co/GYjyMLiqUS">pic.twitter.com/GYjyMLiqUS</a> —@CENTCOM

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

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

U.S. officials say intelligence suggests ISIS is based overwhelmingly in Nangarhar and neighbouring Kunar province.

Estimates of its strength in Afghanistan vary. U.S. officials have said they believe the movement has only 700 fighters, but Afghan officials estimate it has about 1,500.

ISIS's offshoot in Afghanistan is suspected of carrying out several attacks on minority Shia Muslim targets.

The Afghan Taliban, who are trying to overthrow the U.S.-backed government in Kabul, are fiercely opposed to ISIS and the two groups have clashed as they seek to expand territory and influence.