White House press secretary Sean Spicer told reporters on Thursday after the US military deployed the largest nonnuclear bomb in its inventory on an ISIS target in a remote part of far northeast Afghanistan that the US "takes the fight against ISIS very seriously."

The Pentagon announced Thursday that the US dropped the GBU-43/B Massive Ordnance Air Blast, also known as the "Mother of All Bombs" or "MOAB," over the Achin district of Afghanistan's Nangarhar province, which borders northwest Pakistan.

"We targeted a system of tunnels and caves that ISIS fighters used to move around freely, making it easier for them to target US military advisers and Afghan forces in the area," Spicer said at a press briefing. "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 the military "took all the precautions necessary to prevent civilian casualties and collateral damage as a result of the operation."

Spicer referred other questions on the MOAB to the Defense Department. He declined to say whether President Donald Trump personally authorized the use of the bomb.

Adam Lowther, the director of the US Air Force's School of Advanced Nuclear Deterrence Studies, told Business Insider that the bomb was "not even close to being a nuclear weapon" and that he would "not make the argument that it's a symbol of escalation" in the conflict in Afghanistan.