36 Islamic State militants have been killed by a US bomb which was dropped on IS-held caves in Afghanistan, according to reports.

Afghan officials have ruled out any civilian casualties after the bomb, which they say decimated a deep tunnel complex used by the Islamic State.

Afghan Defence Ministry spokesman Dawlat Waziri said: "No civilian has been hurt and only the base which Daesh used to launch attacks in other parts of the province was destroyed."

It is the first time the "mother of all bombs" 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.

Achin District Governor Esmail Shinwari said: "The explosion was the biggest I have ever seen. Towering flames engulfed the area.

"We don't know anything about the casualties so far but, since it is a Daesh stronghold, we think a lot of Daesh fighters may have bee killed."

Afghan government spokesman Shah Hussain Murtazawi said: "Heavy casualties have been inflicted on the enemy."

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 and it is regarded as particularly effective against clusters of targets on or just underneath the ground.

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'.

'Maximising destruction'

The weapon, which was first tested just days before the Iraq War in March 2013, was dropped at 7.32pm local time (4.02pm Irish 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.

US President Donald Trump praised the US military for carrying out another "successful" mission, but was both vague and threatening when asked if the use of the weapon carried an implicit warning to North Korea.

He said: "I don't know if this sends a message to North Korea... North Korea is a problem, the problem will be taken care of."