JALALABAD - Afghan forces backed by US airstrikes Tuesday ramped up an offensive against the Islamic State group in their eastern stronghold, days after the jihadists claimed the deadliest attack for 15 years in the capital Kabul.

Afghan troops seized large parts of the mountainous district of Kot in Nangarhar province, a key IS stronghold where the jihadists set up sharia courts and training camps displaying their trademark black flag.

Some 120 militants were killed in the operation including Saad Emarati, a prominent local IS commander, provincial spokesman Attaullah Khogyani told AFP.

"They have been taught a lesson for their crimes. We will wipe out IS from Nangarhar," said defence ministry spokesman Mohammad Radmanish.

The offensive follows the twin bombings on Saturday that tore through crowds of minority Shiite Hazaras as they protested over a major power line in Kabul, killing 80 people and leaving hundreds of others maimed.

IS, which controls territory across Syria and Iraq, claimed the bombings in a statement carried by its affiliated Amaq news agency, calling it an attack on a "gathering of Shiites".

The group said the attack was in retaliation for the support offered by some Hazaras to the regime of Syrian President Bashar al-Assad.

Many Hazaras have been covertly recruited by Shiite-dominated Iran, Assad's key regional ally, to fight for his government against IS.

The devastating attack in the capital represents a major escalation for IS, which so far has largely been confined to Nangarhar where it is notorious for brutality including beheadings.

It also raised the spectre of sectarian discord in Afghanistan, something that the Sunni majority country has largely been spared despite decades of war.

But officials have denied that it marked a turning point for IS in Afghanistan, saying the group has been under heavy pressure from both US air strikes and a ground offensive led by Afghan forces .

US military spokesman Charles Cleveland said the group has steadily lost territory, with fighters largely confined to two or three districts in Nangarhar from around nine in January.

He said US forces have conducted more than 450 airstrikes since the beginning of the year.

The NATO-led coalition estimates there are between 1,000 and 3,000 IS militants in Afghanistan - mostly disaffected Pakistani and Afghan Taliban, as well as Uzbek militants and local residents of Nangarhar.

The offensive marks the start of a new chapter in the country's war against insurgents.

While the main fight is against the Taliban, a stronger group than IS, the campaign is a departure from the previous defensive strategy of Afghan forces , who have struggled to contain insurgents since US-led NATO troops ended their combat mission in 2014.

US forces were last month granted greater powers to strike at militants, as President Barack Obama vowed to take the fight to the insurgents in a more aggressive campaign.