Former premier Dr Mahathir Mohamad has insisted that he now has nothing against jailed former opposition leader Anwar Ibrahim.

This comes amidst calls for him to support demands for Anwar's release from his Sungai Buloh prison cell as part of the Citizens' Declaration movement.

However the long-running premiere in an interview with Al Jazeera English's current affairs show ‘UpFront’ said he is unrepentant over having removed Anwar as his deputy in 1998.

"I have no power to order anybody to be released... I have nothing against him now. It was before," said Mahathir in the interview to be aired later tonight.

"His detention now is not because of me, it's because of Najib," he said, in reference to prime minister Najib Abdul Razak.

Mahathir in 1998 sacked Anwar as the then deputy prime minister and education minister, triggering the first Reformasi movement that followed his first incarceration for abuse of power.

Anwar is currently serving a five-year jail sentence after he was convicted on Feb 10, 2015, for sodomy in relation to his former aide Mohd Saiful Bukhari Azlan.

In the interview, Mahathir also commented on his own political future, after quitting Umno and leading the call for Najib's resignation.

"I’m not ruling out anything. Anything is possible," he said to a question on whether he might consider rejoining Umno.

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Similarly, Mahathir refused to rule out one of his sons becoming prime minister as a means to avoid any conflict of interest.

"I don't rule out anything. I have no right to rule out anything. Anybody who wants to make a bid for whatever, he is entitled to do it," he said.

During the recently concluded twin by-elections, Mahathir had campaigned for Pakatan Harapan candidates in both Sungai Besar and Kuala Kangsar, calling for voters to vote against BN as a "referendum" on the Najib's administration

However, with BN's victory in both seats, critics have challenged Mahathir to respect its outcome as an affirmation of the PM.