Shahidullah Shahid, a spokesman for the Pakistan Taliban, has been sacked after pledging his allegiance to the Islamic State.

The news follows the release of an audio message in which Shahid appears to state that he and a group of Taliban commanders were defecting to the jihadist group bidding to carve out an Islamic caliphate in the Middle East.

The authenticity of the recording, which was posted online last week, has yet to be independently verified. Shahid's cell phone has reportedly been switched off ever since its release.

The Pakistan Taliban (TTP) said in a statement that he had been replaced but did not announce the name of his successor. The group reaffirmed its support for Mullah Mohammed Omar, the commander of the Afghanistan Taliban.

"As far as allegiance to ISIS is concerned (TTP leader) Amir Mullah Fazlullah has clarified that our allegiance is to Mullah Mohammad Omar Mujahid," the militants said, according a translation by Reuters.

Along with Shahid, five Pakistan Taliban commanders are understood to have defected to IS, which controls swathes of both Syria and Iraq. It has come into conflict with al-Qaeda and some other militant organizations, though some smaller groups such as Algeria's Jund al-Khalifah (Soldiers of the Caliphate) have answered its call for support and carried out kidnappings and executions of hostages at its behest.

The statement from the Pakistani militants revealed that Shahidullah Shahid was in fact a pseudonym and that the real name of its former spokesman was Sheikh Maqbool.

The TTP leaders who have defected were operating within the Orakzai, Kurram and Khyber tribal regions and in addition the districts of Hangu and Peshawar. Saeed Khan, a commander from Orakzai is viewed as the biggest loss to the Taliban, according to analysts.

A Taliban commander told Reuters that Shahid's decision to pledge allegiance to the Islamic State was part of a planned move designed to grab media attention.

"He used our name and tried to make it big in the media," the commander commented. The Pakistani Taliban has been divided over the past few years, with splinter groups often looking to gain power and influence. Militants based within the Mohmand region for example have split from the Taliban and formed a completely separate group know as the Jamatt-ul-Ahrar.

In addition the powerful Mehsud tribe has refused to bow to the authority of the Pakistani Taliban's new leader Mullah Fazlullah, who was appointed leader after his predecessor Hakimullah Mehsud was killed in a US drone strike last November. The death of the commander had already set off a string of desertions.

Shahid's apparent defection to the Islamic State came barely two weeks after the TTP pledged to support IS by providing it with a stream of fighters. "All Muslims the world have great expectations of you…we are with you, we will provide you with Mujahideen and with every possible support," a statement from the Pakistani militants read.

In June of this year the insurgents were put under further pressure by a massive army offensive in the group's bastion of North Waziristan, an operation launched in response to a deadly assault on Karachi Airport.

Saifullah Mehsud, a researcher at the Pakistani think tank Fata Research Centre, told the BBC that the defections of Shahid and other commanders underscored the fractures in the organization.

"The decision to leave TTP and join IS brings to the fore the extent to which the militants are divided now. The organisation has never been more in dire straits than now," he said.

IS had no structure or presence in Pakistan, he said, but the group could nevertheless fuel further splits in the Taliban, he said.

"There can be only one leader — either Mullah Mohammad Omar or (Islamic State leader) Abu Bakr al-Baghdadi."