Malaysia's nearly 100-year-old former prime minister Mahathir Mohamad is being probed in a graft investigation involving his two sons, the head of the country's anti-corruption body said Thursday.

The Malaysian Anti-Corruption Commission (MACC) in January ordered Mahathir's eldest son, Mirzan Mahathir, to declare all his assets in and outside the country as part of an investigation following revelations contained in the so-called Panama Papers.

MACC's chief commissioner Azam Baki told reporters in the northeastern state of Kelantan on Thursday that Mahathir was also being probed.

"I wish to confirm that he (Mahathir) is among those being investigated. This follows an investigation into his sons," he said in a televised remarks.

"His two sons were issued with notices to declare their assets," he added, declining to provide details.

An aide to Mahathir told AFP on condition of anonymity that "no formal notice has been issued against Mahathir informing the former premier of any investigation".

Mahathir, 98, is a political rival and fierce critic of Prime Minister Anwar Ibrahim, who came to power in 2022 vowing to stamp out high-level corruption.

Anwar has denied accusations of using the MACC investigation to go after his political foes.

MACC said it started probing in August 2022 "entities linked and named" in the Pandora and Panama papers, which shone a light on tax evasion by the rich and powerful around the world.

In February, MACC charged former finance minister Daim Zainuddin and his wife for failing to disclose assets. Daim is a close associate of Mahathir.