2017-04-13 20:14:45 UTC
It is the first time this type of bomb has been used in combat, Pentagon spokesman Adam Stump said.
It was dropped from an MC-130 aircraft in the Achin district of Nangarhar province, very close to the border with Pakistan.
The GBU-43 bomb, weighing 21,600 lbs (9,797kg), is GPS-guided, contains 11 tonnes of explosives and is the US military's largest non-nuclear device.
Its nickname, 'the mother of all bombs', is based on the name given to it by the US Air Force - 'Massive Ordnance Air Blast' bomb.
Mr Stump said the bomb was dropped on a group of caves and bunkers, believed to be used by Islamic State in Afghanistan, an affiliate group known by its US military acronym of 'ISIS-K'.
The weapon, which was first tested in March 2003, just days before the start of the Iraq War, was dropped at 7.32pm local time (4.02pm UK time) after its deployment was signed off by General John Nicholson, the commander of US forces in Afghanistan.
General Nicholson said the strike was designed to minimise the risk to Afghan and US forces conducting clearing operations in the Achin area "while maximising the destruction" of IS fighters and facilities.
He said IS has been using improvised explosive devices, bunkers and tunnels to strengthen its defences.
In a statement, General Nicholson said: "This is the right munition to reduce these obstacles and maintain the momentum of our offensive against ISIS-K."
White House press secretary Sean Spicer said of the strike: "The GBU is a large, powerful and accurately delivered weapon. We targeted a system of tunnels and caves that ISIS fighters used to move around freely, making it easier for them to target US military advisers and Afghan forces in the area."
The military says it is still assessing the damage it has caused.