TTP spokesman Shahidullah Shahid in a statement confirmed the allegiance to IS and said they would be the IS's lead fighters in Pakistan, Dawn online reported.
"I am confirming my allegiance to Amirul Momineen Abu Bakar Al-Baghdadi and would abide by all his decisions, whatever is the order, and whatsoever the circumstances, I shall be loyal to him and obey his orders," Shaidullah Shahid said in a statement issued in Arabic and Urdu.
He added that TTP chief Mullah Fazlluah has extended his support to Al-Baghdadi but has not pledged allegiance to IS.
Shahidullah said he has already extended his allegiance three times to Al-Baghdadi and it was the fourth time that he and his associates were extending allegiance to the IS chief.
The first time was when Al-Baghdadi declared himself the "caliph" of areas across Iraq and Syria.