The death toll of Islamic State (IS) militants killed when the US military dropped a massive GBU-43 bomb or the "mother of all bombs" in Afghanistan's Nangarhar province has risen to 94, a Kabul official said on Saturday.

"The number of IS militants killed in the US bomb in Achin district jumped to 94, including four commanders," Nangarhar provincial spokesman Attaullah Khogiani told CNN.

"Our team is in the area and they are doing clearance, so the figure might change as they find more bodies," said Dawlat Waziri, a spokesman for the Afghan Ministry of Defence.

The GBU-43, a non-nuclear, 10-tonne missile powered by a wave of air pressure, was dropped on Thursday onto caves used by the terror group.

The initial toll given by Afghan officials for the strike was 36. However, a statement released on Friday through IS' media wing, Amaq News Agency, said none of the terror group's fighters were killed or injured. 

No civilians were killed in the explosion, said an official. 

The Chief Executive of Afghanistan, Abdullah Abdullah, said the attack had been carried out in co-ordination with his government and "great care had been taken to avoid civilian harm". 

The strike targeted a network of fortified underground tunnels that IS had been using to stage attacks on government forces in Nangarhar province, near the Pakistan border, said the report.

The blast destroyed three underground tunnels as well as weapons and ammunition, but no civilians were hurt, Afghan and US officials said.

The US military defended its decision when it was quizzed Friday on whether the behemoth bomb was necessary for that particular target.

"This was the right weapon against the right target. It was the right time to use it tactically against the right target on the battlefield," General John Nicholson, commander for US forces in Afghanistan, said at a news conference.

"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 southern Nangarhar."

The US military previously estimated IS had 600 to 800 active fighters in the area.

Afghan President Ashraf Ghani said he approved of the strike, and it was designed to support Afghan and US forces conducting clearance operations in the region.

But former President Hamid Karzai accused the US of using Afghanistan as "a testing ground for new and dangerous weapons."