Helicopters poured water for hours on the mountains around Nanyo city in Yamagata prefecture to douse the flames, but the operation was called off after dark with the fire still burning.

"We're urging over 400 residents in the three districts" of the city to evacuate, an official in charge of the disaster response told AFP.

"There is a risk the fire will spread" to the residential area, she said, adding firefighting efforts will resume on Monday morning.

Footage from public broadcaster NHK showed white smoke rising from the mountains.

The northern city has been sweltering under higher-than-average temperatures and saw the mercury rise past 31 degrees on Sunday, according to private weather forecaster Weather News.

The fire broke out on Saturday -- prompting several road closures -- with around 95 hectares of land burnt down by Sunday evening, city authorities said.

Globally, 2024 has already been marked by climate extremes and rising greenhouse emissions, spurring fresh calls for more rapid action to limit global warming.

Japan's northern Sapporo city, known for its February snow festival, saw temperatures hit 25 degrees Celsius last month, the earliest since records began in 1877.