The U.S. military said it had degraded the number of Islamic State (ISIS/ISIL) fighters in Afghanistan by an estimated 80 percent to about 600 jihadists before dropping the largest non-nuclear bomb on the terrorist group on Thursday.

“As ISIS-K’s losses have mounted, they are using [improvised bombs], bunkers and tunnels to thicken their defense,” said Army Gen. John W. Nicholson, top commander of U.S. and NATO forces, after the U.S. strike on the jihadist group in Afghanistan.“This is the right munition to reduce these obstacles and maintain the momentum of our offensive against ISIS-K.”

The Pentagon confirmed that the U.S. military dropped the “mother of all bombs,” officially known as the GBU-43/B Massive Ordnance Air Blast, on an Islamic State tunnel complex in Achin district, located in eastern Afghanistan’s Nangarhar province, marking the first time the massive 21,600-pound bomb was used in battle.

U.S. military officials have identified eastern Afghanistan’s opium-rich Nangarhar province, located along the country’s border with Pakistan, as the group’s primary stronghold in the region.

ISIS officially established a branch in the Afghanistan-Pakistan region, dubbed the Khorasan Province (IS-KP/ISIS-K) in January 2015, less than a month after former President Barack Obama and NATO announced an end to their combat mission in Afghanistan.

The end of combat declaration came with a substantial reduction of coalition forces, allowing the number of ISIS-K fighters to reach 3,000 at its peak.

Last October, Gen. John Nicholson, the top commander of U.S. and NATO forces in Afghanistan, warned that ISIS was intent on establishing a caliphate in the Afghanistan region.

“Right now we see them very focused on trying to establish their caliphate, the Khorasan caliphate, inside Afghanistan,” Gen. Nicholson told NBC News.

However, the U.S. military recently said the group’s manpower has been diminished by an estimated 80 percent, to 600 jihadists.

Most of the ISIS-K jihadists are former members of Pakistani Taliban, or Tehrik-e Taliban, and its Afghan counterpart, according to the Pentagon.

“Our goal in 2017 is to defeat ISIS-K in Afghanistan,” American Navy Capt. Bill Salvin, a top spokesman for the U.S./NATO-led coalition in Kabul, told Voice of American (VOA) at the end of last month.

“In 2016, we believed that year began with about 3,000 or so ISIS-K members in about 12 districts in southern Nangarhar,” added the captain. “Right now, we believe there are about 600 ISIS-K members in two or three districts in southern Nangarhar.”

The Trump administration has taken steps to combat ISIS-K that appear to be paying off.

President Trump has granted military commanders, including Gen. Nicholson, “broader latitude to act independently,” notes CNN.

Without explicitly saying whether he personally approved Thursday’s strike on ISIS in Afghanistan, Trump declared, “Everybody knows exactly what happens. So, what I do is I authorize our military.”

“We have given them total authorization and that’s what they’re doing,” he added, referring to U.S. commanders.

CNN learned that Gen. Nicholson signed off on the recent strike.

Adam Stump, a Pentagon spokesman, revealed that the authority to deploy to bomb was granted to Nicholson by Gen. Joseph Votel, the chief of U.S. Central Command.

Khorasan is an ancient name for a region that covers parts of Afghanistan, Pakistan, Iran, India, and other neighboring countries.

From the Khorasan region, ISIS-K is also positioned to pose a threat to China, which shares a border with Afghanistan.