KUALA LUMPUR (The Star/Asia News Network) - Malaysia's former prime minister Tun Dr Mahathir Mohamad has been admitted to the National Heart Institute (IJN) for treatment for chest infection.

A statement from IJN said the 91-year-old "is expected to be kept under observation for the next few days".

"Tun is currently admitted in the general ward and based on the medical team's advice, visitation is strictly for close family members only," the statement said.

Mahathir, who was prime minister for 22 years until he retired in 2003, has undergone several heart operations.

The past few months has been busy for him as he was involved in the formation of the new Parti Pribumi Bersatu Malaysia together with former deputy prime minister Muhyiddin Yassin - a fellow critic of current premier Najib Tun Razak.

He was named as chairman of the party on Tuesday in registration papers filed by Muhyiddin with the Registrar of Societies.