Jamaat-ul-Ahrar (JuA), a faction of the Pakistani Taliban, had claimed responsibility for many deadly attacks in the country, including a suicide bombing in a park in Lahore on Easter Sunday last year that killed 75 people including many children.
"Chief of our Jamaat-ul-Ahrar Umar Khalid Khorasani, who sustained serious injuries in a recent US drone strike in Afghanistan's Paktia province, succumbed to his injuries Wednesday evening," JuA spokesman Asad Mansoor told AFP by telephone from an undisclosed location.
"At least nine close associates of Khorasani were also killed in the strike", he added.
Two US drone strikes on Monday along the border between Afghanistan and Pakistan killed at least 26 people including fighters from the Haqqani network which is allied to the Afghan Taliban.
The US has increased pressure on Pakistan in recent months to crack down on the Haqqanis, who are one of the strongest factions in the Afghan Taliban insurgency and have earned a fearsome reputation for their attacks on NATO troops and Afghan installations over the years.
The use of US drones has dwindled dramatically in recent years in Pakistan, where the strikes have proven extremely controversial with the public and rights groups.
Monday's strikes came days after Canadian hostage Joshua Boyle and his American wife and three children were freed in Pakistan after five years of captivity at the hands of the Haqqani network.
Mansoor, the spokesman, said a high-level meeting of JuA's consultative council will be convened soon to appoint the outfit's new chief.
However, an important Taliban commander said on condition of anonymity that one of JuA's senior commanders, Asad Afridi, has already been appointed new JuA chief.
JuA pledged allegiance to IS in 2014, but a year later said it had rejoined the Pakistani Taliban -- which in turn is allied to Al-Qaeda, a foe of IS.
However since 2016 JuA and IS have both laid claim to several attacks, notably a suicide bombing at a hospital in the southwestern city of Quetta last year that killed 73 people.
Pakistan's deadliest ever attack occurred in Peshawar in December 2014, when Taliban militants stormed a school killing more than 150 people, mostly children.
The joint claims raised the possibility the outfits were colluding -- or that JuA, which has foot soldiers on the ground, was allowing IS also to take credit to achieve propaganda goals.
Analysts said Khorasani's death suggested resumption of intelligence sharing and coordination between Pakistan and the US.
"The resumption of drone strikes on anti-Pakistan elements following hard stance on Trump's Afghan policy signals moderation of positions by Islamabad and Washington," leading Pakistan security analyst Imtiaz Gul told AFP.
He said "it shows America's willingness to redress Pakistan's complaints about presence of militant safe havens on Afghan soil".
"Both countries now seem to be back to their business."
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