Pakistani Taliban spokesman Ahsanullah Ahsan was quoted by television news channels as saying that a video of Sultan Amir Tarar alias Colonel Imam, showing him dead, will be released later in the day.
"The video will show Taliban fighters firing at Colonel Imam. Colonel Imam is no more in this world," Ahsan said.
A little-known militant faction called the Asian Tigers in North Waziristan Agency had earlier claimed they had killed the former ISI official. But a media report early this week had said that Imam was alive and in the custody of Taliban fighters.
Colonel Imam was kidnapped by militants affiliated with the Tehrik-e-Taliban Pakistan led by Hakimullah Mehsud in March last year.
The abductors had reportedly demanded Rs 50 crore and the release of some jailed militants for Colonel Imam's freedom. The family of the veteran intelligence officer has said it had received reports about his death last month but there was no confirmation from any independent source.
Colonel Imam was kidnapped with another former ISI official Khalid Khwaja and Asad Qureshi, a British journalist of Pakistani origin.
While Qureshi was released in September after allegedly paying a ransom of Rs 20 crore, Khwaja was executed by his captors in April 2010.
The Asian Tigers had claimed responsibility for the kidnapping and accused Khwaja of spying for the Central Intelligence Agency.
Analysts believe the kidnappers were a group of militants from the Mehsud tribe and Punjabi Taliban rebels led by Sabir Mahsud and Usman Punjabi.
Colonel Imam was widely respected by the Afghan Taliban for his role in the Afghan Jihad against Soviet forces during 1979-89. He described himself as the "teacher" of Afghan Taliban chief Mullah Muhammad Omar in several interviews. He also served as Pakistan's consul general in the western Afghan city of Herat after the installation of a Taliban government in Kabul.