The US military has defended its decision to drop the country's most powerful non-nuclear bomb on ISIS positions in Afghanistan, describing it as a "tactical" move.

The GBU-43/B Massive Ordinance Air Blast bomb (MOAB) was dropped Thursday night on a network of fortified underground tunnels that ISIS had been using to stage attacks on government forces.

The strike, in Nangarhar province near the Pakistan border, also killed 36 ISIS fighters, Afghan officials say.

The US military was quizzed Friday on whether the munition, known as the "mother of all bombs" for its extraordinary force, was necessary for the particular target.

The GPS-guided bomb is capable of destroying an area equivalent to nine city blocks.

"This was the right weapon against the right target," the commander for US forces in Afghanistan, General John Nicholson, told a press conference.

"It was the right time to use it tactically against the right target on the battlefield."

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

The US military previously estimated ISIS had 600 to 800 active fighters in the area, but was unclear whether they had hoped to strike more fighters.

Nicholson gave a vague response to a question by reporters on who exactly ordered or greenlighted the strike, saying only that he enjoyed a certain amount of "latitude" to make decisions in his chain of command.

Nicholson also confirmed the strike was carried out in coordination with Afghan officials, and said that the mission had conducted rigorous surveillance before, during and after the operation to prevent civilian deaths.

"Let me be clear -- we will not relent in our mission to fight alongside our Afghan comrades to destroy ISIS-K in 2017," he said, referring to ISIS' regional branch.

Afghan President Ashraf Ghani said he approved of the strike and that it was designed to support Afghan forces 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."

(Source: CNN)