Rescuers are scrambling to help dozens of hikers stranded on an erupting volcano in central Japan with six people believed to be buried under ash and dozens injured.

Columns of thick white steam were rising on Sunday from the 10,121ft Mount Ontake, which erupted around noon the day before, spewing ash, rocks and steam on otherwise a sunny autumn weekend busy with tourists and hikers.

Seven people were unconscious and buried under ash, national broadcaster NHK said.

One of them was rescued but the others remained on the mountain, NHK said, adding that a total of 42 people were believed to have suffered injuries.

Local media on Saturday reported that a person died, but firefighters said the death had not been confirmed.

A blanket of ash up to 20cm deep covered a large area of the volcano, trapping climbers and forcing up to 150 to seek refuge in mountaintop shelters at one point.

Local officials believe 45 to 49 hikers sought shelter overnight in cabins on the popular mountain.

A group of 23 hikers who spent the night in a cabin on Sunday were able to climb down to reach the start of a trail leading to the summit.

'Two people waving'

Mount Ontake, about 210km west of Tokyo, sits on the border of Nagano and Gifu prefectures, on the main Japanese island of Honshu.

A Self Defence Force helicopter rescued a man and a woman near the summit, according to a spokesman at Otaki village in Nagano.

"The helicopter flew over there very early in the morning to survey the condition. Then it found the two people waving at it," the spokesman told AFP news agency.

"Originally, the rescuers thought it might be difficult to go near them because ashes could rise [and damage the helicopter], but the conditions were better than they believed and they were able to rescue the two people."

The two were able to walk unassisted, but were transported to a hospital for observation, the spokesman said.

About 230 hikers were able to make it to safe ground on Saturday after the eruption.

Television footage showed a line of rescue workers, wearing orange uniforms or green camouflage, scaling grey, ash-covered trails on Sunday.

Hot volcanic ash

In a YouTube video, climbers can be seen moving quickly away from the peak as an expanding plume of ash emerges above and then engulfs them.

Among the injured, some were hit by flying rocks or suffered burns after inhaling hot volcanic ash, the Yomiuri Shimbun said.

Two Jetstar flights headed to Tokyo's Narita International Airport on Saturday diverted to Kansai International Airport in western Japan as a precaution.

The meteorological agency forecast further eruptions, warning that volcanic debris may settle within 4km of the peak.

The agency also placed restrictions on access to the mountain, while calling on local residents to remain alert as an eruption could shatter windows miles away.

The last significant eruption of Mount Ontake was in 1979 when it expelled more than 200,000 tonnes of ash, according to local media.

Source: Agencies