A Japanese volcano erupted on Saturday, leaving seven people unconscious, eight seriously injured and more than 250 stranded.

A thick, grey cloud of ash rose into the sky above Mount Ontake near where TV footage showed hikers taking pictures, according to Reuters.

Trekkers and residents were warned of falling rock and ash within a radius of 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 volcano, which straddles Nagano and Gifu prefectures 125 miles west of Tokyo, erupted just before midday and sent ash pouring down the mountain's south slope for two miles, said The Meteorological Agency.

So far, there was no sign of lava from the TV footage.

Aircrafts were forced to diver their routs because of the eruption, but officials at Tokyo's Haneda airport and Japan Airlines said there were no disruptions to flights in and out of Tokyo.

The mountain is 3,067 metres high and last erupted in 2007, according to Reuters.

"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," Prime Minister Shinzo Abe, who returned from the United States on Saturday, said to reporters.

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

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

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, said to 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."

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