Flames are estimated to have spread over about 1,200 hectares (3,000 acres) in the forest of Ofunato in the northern region of Iwate since fire broke out on Wednesday, according to the Fire and Disaster Management Agency.

"We're still examining the size of the affected area, but it is the biggest since the 1992 wildfire" in Kushiro, Hokkaido, an agency spokesman told AFP.

That fire burned 1,030 hectares, the previous record.

Around 1,700 firefighters were being mobilised from across the country, the agency said.

Aerial footage from public broadcaster NHK showed white smoke billowing up and covering an entire mountain.

Local police found the body of one person who had been burned on Thursday.

Over 1,000 nearby residents have been evacuated and more than 80 buildings had been damaged as of Friday, according to the Ofunato municipality.

The cause of the blaze remained unknown.

Two other fires were also burning Saturday, one in Yamanashi and another elsewhere in Iwate.

There were about 1,300 wildfires across Japan in 2023, concentrated in the February to April period when the air dries out and winds pick up.

The number of wildfires has declined since the peak in the 1970s, according to government data.

Ofunato has seen only 2.5 millimetres (0.1 inches) of rainfall this month -- on course to fall far below the previous record low for February of 4.4 millimetres in 1967.

And last year was Japan's hottest since records began, mirroring other nations as ever-rising greenhouse gas emissions fuel climate change.