More than 500 Japanese military and police have set out to search the peak of a volcano popular with hikers a day after its sudden eruption trapped hundreds on the mountain.

Japanese media said seven people were reported to be unconscious, possibly buried in ash, as Mount Ontake continued to spew smoke and ash into the sky, while local officials said they were trying to confirm the whereabouts of 32 hikers.

People have been evacuated from the mountain after the eruption. ( AFP: Jiji Press )

Late on Saturday, Japanese media reported one woman was dead but that was later withdrawn. At least 40 people were injured, including several with broken bones, officials said.

"It's very hard to know what's happening on the mountain now and things could change," said one official with the government of Nagano prefecture, one of two prefectures straddled by the 3,067 metre volcano.

Hundreds of people, including children, were stranded on the peak after it erupted without warning just before noon on Saturday, sending ash pouring down the slope for more than three kilometres.

Most people made their way down that evening but some 30 were still stranded on Sunday morning.

Video footage posted on the internet showed huge grey clouds boiling towards climbers at the peak and people scrambling to descend as blackness enveloped them.

"All of a sudden ash piled up so quickly that we couldn't even open the door," Shuichi Mukai, who worked in a mountain hut just below the peak, said.

The hut quickly filled with hikers taking refuge.

"We were really packed in here, maybe 150 people. There were some children crying, but most people were calm. We waited there in hard hats until they told us it was safe to come down."

The mountain, some 200 kilometres west of Tokyo, is a popular site to view autumn foliage, currently at its best.

Flights at Tokyo's Haneda airport suffered delays as planes changed routes to avoid the peak but were mostly back to normal by Sunday, an airport spokeswoman said.

An official at the volcano division of the Japan Meteorological Agency said while there had been a rising number of small earthquakes detected at Ontake since September 10, the eruption could not have been predicted easily.

"There were no other signs of an imminent eruption, such as earth movements or changes on the mountain's surface," the official told Reuters. "With only the earthquakes, we couldn't really say this would lead to an eruption."

Reuters

Loading

Loading