The GBU-43/B Massive Ordinance Air Blast bomb (MOAB) was dropped Thursday night on a network of fortified underground tunnels that ISIS had been using to stage attacks on government forces.
The strike, in Nangarhar province near the Pakistan border, also killed 36 ISIS fighters, Afghan officials say.
The US military was quizzed Friday on whether the munition, known as the "mother of all bombs" for its extraordinary force, was necessary for the particular target.
The GPS-guided bomb is capable of destroying an area equivalent to nine city blocks.
"This was the right weapon against the right target," the commander for US forces in Afghanistan, General John Nicholson, told a press conference.
"It was the right time to use it tactically against the right target on the battlefield."
The blast destroyed three underground tunnels as well as weapons and ammunition, but no civilians were hurt, Afghan and US officials have said.
The US military previously estimated ISIS had 600 to 800 active fighters in the area, but was unclear whether they had hoped to strike more fighters.
Nicholson gave a vague response to a question by reporters on who exactly ordered or greenlighted the strike, saying only that he enjoyed a certain amount of "latitude" to make decisions in his chain of command.
Nicholson also confirmed the strike was carried out in coordination with Afghan officials, and said that the mission had conducted rigorous surveillance before, during and after the operation to prevent civilian deaths.
"Let me be clear -- we will not relent in our mission to fight alongside our Afghan comrades to destroy ISIS-K in 2017," he said, referring to ISIS' regional branch.
Afghan President Ashraf Ghani said he approved of the strike and that it was designed to support Afghan forces and US forces conducting clearance operations in the region.
The US bomb was dropped as Washington comes under increased scrutiny over its military actions in the Middle East, including three US-led airstrikes in the past month that are reported to have killed civilians or allies.
But US President Donald Trump said Thursday that the Afghanistan bombing was "another successful job."
Residents in villages kilometers away from the target area felt Thursday's powerful strike as if bombs had fallen nearby.
A local resident living around two kilometers (1.5 miles) from the blast told CNN he heard an "extremely loud boom that smashed the windows of our house."
"We were all scared and my children and my wife were crying. We thought it had happened right in front of our house," he said.
"I have witnessed a countless number of explosions and bombings in the last 30 years of war in Afghanistan, but this one was more powerful than any other bomb as far as I remember."
Another Afghan man, 46-year-old Abdul, who lives three kilometers from the site, described the thick cloud of dust that formed after the deafening blast.
"We were unable to see each other at home because of the excessive dust inside the room," he said.
"I was feeling that boom 'til the morning."
Locals told CNN that more than 3,000 families had fled the district in the past year or so since the militant group established its presence.
Those troops are separate to a wider NATO-led effort to train, advise and assist the Afghan army and police force.
The Taliban "control or contest" about a third of the population of the country, Bergen said, citing senior US military officials. That's around 10 million people -- more than the population ISIS controlled in Syria and Iraq at the height of its power during the summer of 2014, he added.
This is the first time a MOAB has been used in the battlefield, according to the US officials. The munition was developed during the Iraq war and is an air blast-type warhead that explodes before hitting the ground in order to project a massive blast to all sides.
Military officials said they hoped the MOAB would create such a huge blast that it would rattle Iraqi troops and pressure them into surrendering or not even fighting.
As originally conceived, the MOAB was to be used against large formations of troops and equipment or hardened above-ground bunkers. The target set has also been expanded to include targets buried under softer surfaces, like caves or tunnels.
CNN's Angela Dewan, Ehsan Popalazia, Ryan Browne, Zachary Cohen, Jim Acosta, Jeremy Diamond, Ehsan Popalzai and Euan McKirdy contributed to this report.
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US military defends dropping 'mother of all bombs' on ISIS in Afghanistan - CNN