A Japanese volcano erupted on Saturday, spewing ash and small rocks into the air and leaving seven people unconscious, eight seriously injured and more than 250 stranded on the mountain, officials and media said.

A thick, rolling, grey cloud of ash rose into the sky above Mount Ontake close to where TV footage showed hikers taking pictures. Trekkers and residents were warned of falling rock and ash within a radius of four km (two and a half miles).

"It was like thunder," a woman told broadcaster NHK of the first eruption at the volcano in seven years. "I heard boom, boom, then everything went dark."

The Meteorological Agency said the volcano, which straddles Nagano and Gifu prefectures 200 km (125 miles) west of Tokyo, erupted just before midday and sent ash pouring down the mountain's south slope for more than three km (two miles).

There was no sign of lava from the TV footage.

The eruption forced aircraft to divert their routes, but officials at Tokyo's Haneda airport and Japan Airlines said there were no disruptions to flights in and out of Tokyo.

NHK quoted a Nagano prefectural official as telling a government meeting that seven people were unconscious and eight people were seriously wounded.

Police said more than 250 hikers were stranded on the mountain, which is 3,067 metres high and last erupted in 2007.

Prime Minister Shinzo Abe, who returned from the United States on Saturday, said he had issued instructions to mobilise the military to help in the rescue effort.

"Nearly 200 people are in the process of descending the mountain, but we are still trying to figure out details. I instructed to do all we can to rescue the people affected and secure the safety of the trekkers," Abe told reporters.

Nagano police sent a team of 80 to the mountain to assist the climbers who were making their way down, while Kiso Prefectural Hospital, near the mountain, said it had dispatched a medical emergency team.

"We expect a lot of injured people so we are now getting ready for their arrival," said an official at the hospital.

More than five hours after the initial eruption, the thick ash cloud showed no signs of abating, NHK TV showed.

"It's all white outside, looks like it has snowed. There is very bad visibility and we can't see the top of the mountain," Mari Tezuka, who works at a mountain hut for trekkers, told Reuters.

"All we can do now is shut up the hut and then we are planning on coming down... This is a busy season because of the changing autumn leaves. It's one of our busiest seasons."