The top US military commander in Afghanistan says the decision to use a powerful bomb in the country was based purely on tactical considerations.

Gen John Nicholson said the most powerful non-nuclear bomb ever used by the US in combat had been the right weapon to target a suspected Islamic State (IS) base in Nangarhar province.

A 300m (328yds) long network of tunnels and caves was destroyed, he said.

The US coordinated the attack with Afghanistan, Afghan officials said.

About 36 suspected militants were killed, according to Afghan officials, but IS has denied suffering any casualties.

Known as the "mother of all bombs", or MOAB, the device was dropped on Thursday evening by an MC-130 transport plane, falling in Nangarhar's Achin district.

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".

When IS announced the establishment of its Khorasan branch - an old name for Afghanistan and surrounding areas - in January 2015, it was the first time the group had officially spread outside the Arab world.