The claim appears to confirm analysts' predictions that the Middle East based group, which had previously struggled to gain traction in Pakistan, is now building links with local outfits as its key rival Al Qaeda loses ground.
Pakistani authorities said the attack on the Balochistan Police College, the deadliest assault on a security installation in the country's history, was carried out by the Al-Alami faction of the Lashkar-e-Jhangvi (LeJ) group, an anti-Shiite outfit that has turned its guns on the government in recent years.
Asked whether the two had worked together, Ali bin Sufyan, a spokesman for LeJ Al-Alami, told AFP "Yes, of course," adding "We are ready to work with all the groups in Pakistan wether they belong to ISIS or Al Qaeda".
LeJ is also affiliated with the Pakistani Taliban who in turn pledge allegiance to Al-Qaeda -- their main financial backers during the early years of their insurgency.
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"The militant groups including Al Qaeda and Pakistani Taliban have been completely dismantled and those left out are now regrouping and attempting to become part of IS," a security official based in the southwestern city of Quetta told AFP requesting not to be named.
"But IS has not nominated any specific group as its franchise or affiliate in Pakistan and different militant groups are trying to use this open window which sometimes creates a competition of sorts," he said.
"Al Qaeda is no longer focused in Pakistan like it used to be, its leaders have either been killed or shifted to Yemen and local jihadists have no better option but to look towards Daesh (IS)."
Pakistan's military last month admitted for the first time that the Islamic State group had a presence in the country but added it had detained hundreds of its militants and prevented them from carrying out major attacks.
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