Afghan authorities Saturday reported a jump in fatalities from the American military's largest non-nuclear bomb, declaring some 90 militant Islamic State (IS) fighters dead, as US-led ground forces advanced on their mountain hideouts.

Dubbed the “Mother Of All Bombs”, the GBU-43/B Massive Ordnance Air Blast was unleashed in combat for the first time Thursday, hitting IS positions in a remote area of eastern Nangarhar province.

The unprecedented attack triggered global shock waves, with some condemning the use of Afghanistan as what they called a testing ground for the weapon, and against a militant group that is not considered a threat as big as the resurgent Taliban.

The bomb smashed IS's hideouts, a tunnel-and-cave complex that had been mined against conventional ground attacks, engulfing the remote area in a huge mushroom cloud and towering flames.

“At least 92 Daesh (IS) fighters were killed in the bombing,” Achin district governor Esmail Shinwari told AFP on Saturday.

Shinwari said that American and Afghan ground forces were slowly advancing on the mountainous area blanketed with landmines, but there were still some pockets of resistance from insurgents.

“New fighters have probably come from the other side of the border to collect the dead bodies,” he added. Nangarhar provincial spokesman Attaullah Khogyani gave a death toll of 90, far higher than the initial toll of 36 IS fighters given by Afghan officials.

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.

An elderly man who lives close to the bombing site in Achin's Momand Dara area said the blast was so piercingly loud that his infant granddaughter was experiencing hearing loss.

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.

“The enemy had created bunkers, tunnels and extensive mine fields, and this weapon was used to reduce those obstacles so that we could continue our offensive in Nangarhar,” General John Nicholson, the top US commander in Afghanistan, said on Friday.

'Disproportionate'

President Ashraf Ghani threw his support behind the bombardment, saying it was “designed to support the efforts of the Afghan National Security Forces (ANSF) and US forces conducting clearance operations in the region.”

The bombing came only a week after US President Donald Trump ordered missile strikes against Syria in retaliation for a suspected chemical attack, and as China warned of the potential for conflict amid rising US tensions with North Korea.

Trump hailed the mission in Achin as “very, very successful”.

But some analysts called the action “disproportionate”.

“The Trump administration made a lot of noise with this bomb, but the general state of play on the ground remains the same: The Taliban continues to wage a formidable and ferocious insurgency. ISIS, by comparison, is a sideshow,” Michael Kugelman of the Woodrow Wilson Center in Washington told AFP, using an alternative acronym for IS.

“Still, from a strategic standpoint, there is an unsettling takeaway here: The US pulled off a huge shock and awe mission against an enemy that isn't even the top threat to the US in Afghanistan. The Taliban continues to sit pretty.”

The Taliban, a much bigger insurgent group, is expected to soon announce the start of this year's fighting season.

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 militants.

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.