Spokesman Shahidullah Shahid said he had accepted the "leadership of the Islamic State chief, Abu Bakr al-Baghdadi", whose group has taken over large swatches of territory in Iraq and Syria.

Shahid said the Taliban commanders of Orakzai, Kurram and Khyber tribal districts, the Taliban chief of Hangu district in Khyber-Pakhtunkhwa province and the militant chief of provincial capital Peshawar had all vowed to follow al-Baghdadi.

"The announcement of allegiance is not by Pakistan Taliban as group, nor by our chief Maulana Fazlullah, but only by me and five other commanders," Shahid said.

He said Fazlullah supported ISIS but had not formally announced allegiance to the group.

Fazlullah more than a week ago promised to support the ISIS militants, but Taliban later dismissed reports that this amounted to a formal declaration of allegiance.

The announcement by the Taliban commanders will come as a blow to al-Qaeda's dominance in the region, as the group depends on local militants for logistics and sanctuaries, an intelligence official said to Reuters.

"The decision has tacit approval by Fazullah who had a secret meeting last month with some Arab fighters in Kunar province of Afghanistan," he said on condition of anonymity.

Maulana Fazlullah

He said it shows that al-Qaeda, which had previously split with IS, is losing ground in Pakistani areas, while Islamic State is seen as the "rising star" of Islamic militancy.

The commanders' announcement comes months into a Pakistan military operation against Taliban strongholds in north-west Pakistan.

According to military so far about 1,100 militants have been killed in the offensive. No figures were given for military casualties.