A volcano in central Japan has erupted, catching mountain climbers by surprise and injuring at least 40 people who were stranded in areas that rescue workers had been unable to reach.

Sohei Hanamura, a crisis management official in Nagano prefecture, said on Saturday that another seven people were missing in the dramatic incident.

Police, fire and military rescue workers were trying to approach the 3,067-metre Mount Ontake area on foot, after deciding that the ash in the air made it too dangerous to use helicopters. The ash was also hampering their ascent.

With a sound likened to thunder, the volcano erupted shortly before noon on a clear day, spewing large white plumes of ash high into the sky and sending people on the mountainside fleeing.

The eruption continued into the night, blanketing the surrounding area in ash. About 250 people were initially trapped on the slopes, but most had made their way down by Saturday night, Japanese public broadcaster NHK reported. Some were in shelters set up in four nearby towns.

Dramatic incident

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.

Mikio Oguro, an NHK journalist who was on the slope on an unrelated assignment, said he saw massive smoke coming out of the crater, blocking sunlight and reducing visibility to zero.

"Massive ash suddenly fell and the entire area was totally covered with ash," he said by phone. He and his crew had to use headlamps to find a lodge.

"My colleagues later told me that they thought they might die," Oguro said.

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

Japan's meteorological agency raised the alert level for Mount Ontake to three on a scale of one to five. It warned people to stay away from the mountain, saying ash and other debris could fall up to four kilometres away.

Mount Ontake, about 210 kilometres west of Tokyo, sits on the border of Nagano and Gifu prefectures, on the main Japanese island of Honshu. The volcano's last major eruption was in 1979.

Source: AP