A 61-year-old man suspected of starting a fire at a Japanese mental health clinic that claimed 25 lives died in hospital on Thursday, local media said. 

Morio Tanimoto had been in critical condition due to burns and carbon monoxide poisoning following the deadly attack in Osaka on December 17. 

Tanimoto, a former patient of the clinic, died without being interrogated by police, public broadcaster NHK said, citing investigative sources.

An Osaka police spokesman declined to comment when contacted by AFP.

The fire raged for half an hour, gutting the fourth floor of the narrow commercial building where the clinic, which also provided general medical care, was located.

Japanese media said most of the victims were believed to have suffered from carbon monoxide poisoning, and may have been trapped inside the unit with the exit blocked.

Some patients saw a man placing a paper bag containing a flammable liquid next to a heater, which he then kicked over to ignite, the reports said.

Osaka in western Japan is a major economic hub and the country's second-biggest metropolis after the greater Tokyo region.

One year ago, a man was charged with murder over a 2019 arson attack on a Kyoto animation studio that killed 36 people, the country's deadliest violent crime in decades.

The attack sent shockwaves through the anime industry and its fans in Japan and around the world.