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.”VIDEOU.S. drops mega-bomb on ISIS caves in AfghanistanThe 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.