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‘Mother of all bombs:’ US drops largest non-nuclear bomb in Afghanistan

WASHINGTON, D.C. -- The US military has dropped an enormous bomb in Afghanistan, according to four US military officials with direct knowledge of the mission.

A GBU-43/B Massive Ordnance Air Blast Bomb (MOAB), nicknamed the "mother of all bombs," was dropped at 7:32 pm local time Thursday, the sources said. A MOAB is a 21,600-pound, GPS-guided munition that is America's most powerful non-nuclear bomb.

The bomb was dropped by an MC-130 aircraft, stationed in Afghanistan and operated by Air Force Special Operations Command, Pentagon spokesman Adam Stump told CNN.

SEE ALSO: US military says ‘Misdirected airstrike’ killed 18 allied Syrian forces battling ISIS

Officials said the target was an ISIS cave and tunnel complex and personnel in the Achin district of the Nangarhar province.

"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," White House press secretary Sean Spicer said later Thursday. The strike "targeted a system of tunnels and cave that ISIS fighters use to move around freely."

The military is currently assessing the damage. Gen. John Nicholson, commander of US forces in Afghanistan, signed off on the use of the bomb, according to the sources. The authority to deploy the weapon was granted to Nicholson by the commander of US Central Command, Gen. Joseph Votel, Stump said.

This is the first time a MOAB has been used in the battlefield, according to the US officials. This munition was developed during the Iraq War.

"As ISIS-K's losses have mounted, they are using IEDs, bunkers and tunnels to thicken their defense," Nicholson said in a statement following the strike.

"This is the right munition to reduce these obstacles and maintain the momentum of our offensive against ISIS-K," Nicholson added.

"US forces took every precaution to avoid civilian casualties with this strike. US Forces will continue offensive operations until ISIS-K is destroyed in Afghanistan," read the statement from US Forces Afghanistan.

The extent of the damage and whether anyone was killed is not yet clear. The military is currently conducting an assessment.

The Pentagon is currently reviewing whether to deploy additional trainers to Afghanistan to help bolster US allies there.

The Achin district is the primary center of ISIS activity in Afghanistan. A US Army Special Forces soldier was killed fighting the terror group there Saturday.

WATCH ABOVE: A misdirected airstrike by the U.S.-led coalition earlier this week killed 18 allied fighters battling the Islamic State group in northern Syria, the U.S. military said Thursday.

On Wednesday, Presiden Donald Trump specifically cited NATO's initiatives in fighting terrorism as a reason for his newfound respect for the organization.

"They made a change, and now they do fight terrorism," Trump said, which was a key demand he as a presidential candidate.

While NATO officials have long stressed that the organization has been combating terrorism for over a decade -- including by fighting and training local troops in Afghanistan -- the alliance has in recent months taken on an even bigger role in counterterrorism.

In February, NATO began training Iraqi troops in country, helping build their capacity for the battle against ISIS. A NATO official told CNN that that effort has "no end date." NATO has also since October flown AWACS surveillance planes in support of the counter-ISIS fight.

"NATO is involved in supporting the fight against terrorism in Syria and Iraq," Jens Stoltenberg told CNN"s Wolf Blitzer following his White House visit. "We are present in the wider Middle East region helping partners like Jordan (and) Tunisia to stabilize their countries and to fight terrorism."

But, he added, "I believe NATO can do more."

General Jens Stoltenberg addresses NATO's role in Afghanistan. WATCH ABOVE:

Defense Spending

During the presidential campaign, Trump repeatedly slammed the members -- 23 out of 28 -- that are not meeting the alliance's recommended defense spending levels of 2% of GDP.

But there has been progress on that front, too.

NATO announced that the its members had boosted its overall non-US defense spending by 3.8%, or $10 billion, in 2016. Romania, Latvia and Lithuania -- all concerned about the ambitions of nearby Russia -- have all recently announced that they will meet the target next year.

Some experts think that Russia's military activities have been a bigger driver of defense spending increases than Trump's pressure, particularly among the alliance's eastern members.

Stoltenberg, however, expressed gratitude to Trump directly Wednesday for his emphasis on boosting allied defense spending.

"I thank you for your attention to this issue," he said. "We are already seeing the effects of your strong focus on the importance on burden-sharing in the alliance."

But the momentum might not last.

"Although (Stoltenberg) also wants Europe to spend more, he's only the political leader of the alliance," Jorge Benitez, the director of NATOSource told CNN. "He can't deliver on increases the way Trump wants him to."

The leaders of Europe and Canada would need to take the hard political decisions to boost defense spending, he said. "I don't think Europeans are taking this threat seriously enough."