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

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.

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.

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.