The number of Islamic State fighters killed by a massive US bomb in eastern Afghanistan has nearly tripled to at least 90, Afghan officials said today.

The GBU-43/B Massive Ordnance Air Blast bomb -- dubbed the "Mother Of All Bombs" -- was unleashed in combat for the first time, hitting IS positions in eastern Nangarhar province on Thursday.

The bomb smashed their mountain hideouts, a tunnel-and-cave complex that had been mined against conventional ground attacks, engulfing the remote area in towering flames.

"At least 92 Daesh (IS) fighters were killed in the bombing," Achin district governor Esmail Shinwari told AFP on Saturday. Nangarhar provincial spokesman Attaullah Khogyani gave a toll of 90.

Afghan officials had earlier said the bombing had killed 36 IS fighters.

Shinwari insisted there were "no military and civilian casualties at all".

Security experts say IS had built their redoubts close to civilian homes, but the government said thousands of local families had already fled the area in recent months of fighting.

The massive bomb was dropped after fighting intensified over the past week and US-backed ground forces struggled to advance on the area. An American special forces soldier was killed last Saturday in Nangarhar while conducting anti-IS operations.

President Ashraf Ghani threw his support behind the bombardment.

But some officials close to him condemned the use of Afghanistan as what they called a testing ground for the weapon, and against a militant group that controls only a tiny sliver of territory and is not considered a huge threat.

IS, notorious for its reign of terror in Syria and Iraq, has made inroads into Afghanistan in recent years, attracting disaffected members of the Pakistani and Afghan Taliban as well as Uzbek Islamists.

But the group has been steadily losing ground in the face of heavy pressure both from US air strikes and a ground offensive led by Afghan forces.