With a sound likened to thunder, Mount Ontake erupted shortly before noon, spewing large white plumes of ash high into the sky and sending people on the mountainside fleeing, covering some in ash.

More than 250 people were trapped on the mountain and a nearby peak in areas that were dangerous to approach, though some had decided to try making their way down as sunset approached, said Nagano prefecture crisis management official Minoru Kashiwabara.

Eleven people were injured, eight seriously, including the seven who lost consciousness, Kashiwabara said.

On the Gifu prefecture side of the mountain, 52 people were able to descend, the Fire and Disaster Management Agency said.

The 3,067-meter (10,062-foot) Mount Ontake sits on the border of Nagano and Gifu prefectures, on the main Japanese island of Honshu.

One witness told Japanese public broadcaster NHK that the eruption started with large booms that sounded like thunder.

In a YouTube video shown on Japanese TV, shocked 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, told the station that 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 headlights to find a lodge to take refuge.

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

Japan's meteorological agency raised the alert level for Mount Ontake to 3 on a scale of 1 to 5. It warned people to stay away from the mountain, saying ash and other debris could fall up to 4 kilometers (2.5 miles) away.