Sicilian Mafia boss Matteo Messina Denaro is "seriously ill", a doctor at the health clinic where the fugitive was captured this week said.

Messina Denaro, 60, a convicted killer, was caught during a visit to the clinic on Monday following 30 years on the run, after being forced to seek treatment for cancer.

"He is seriously ill. The disease has accelerated in recent months," Vittorio Gebbia, head of the oncology department at the Maddalena clinic in Palermo, told the Repubblica daily.

Police continue to search for clues as to how Messina Denaro managed to elude capture for three decades.

This morning, they discovered a bunker-like hideout within another home, not far from an apartment in the small town of Campobello di Mazara where he had most recently been living.

Authorities were searching the hideout, which appeared to be separated by a wall from the rest of the residence, according to Repubblica.

Messina Denaro underwent surgery for colon cancer in 2020 and 2022 under a false name, according to leaked medical records published in Italian media.

He was detained on Monday after detectives discovered through wiretapped conversations with family members that he was ill.

Matteo Messina Denaro was captured after 30 years on the run

They then searched Italy for possible suspects of the right gender and age with the same type of cancer.

The officers checked with Dr Gebbia whether Messina Denaro needed urgent treatment.

"The police asked me if it mattered if the chemotherapy cycle he was set to receive was delayed by a few days, and I signed off on it because such a small delay will have no effect," he said.

Messina Denaro was moved shortly after his arrest in Palermo to a high security prison in L'Aquila in the Abruzzo region, where he was being held in solitary confinement.

He was expected to be taken for chemotherapy treatment at the San Salvatore hospital in L'Aquila, which has a special unit reserved for this type of prisoner, according to the Corriere della Sera daily.