A Japanese military helicopter rescued seven people Sunday morning from a spectacular volcanic eruption that sent officials scrambling to reach many more injured and stranded on a mountain.

Mount Ontake in central Japan erupted shortly before noon Saturday, catching mountain climbers by surprise and injuring at least 34, including 12 seriously, according to Japan's Fire and Disaster Management Agency. The tally was lower than reported by local officials earlier, but the disaster agency warned that the numbers could still change.

The number of those reported missing also rose sharply to 32, the agency said.

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Ash from Mount Ontake covered these mountain lodges. The volcanic eruption left more than 250 people stranded near the peak, but most managed to make their way to safety. (Kyodo/Reuters)

‚ÄčNagano prefecture official Sohei Hanamura said that three people were rescued by a military helicopter. Their conditions were not known, he said, and it wasn't clear if they were part of an estimated 40 people stranded at mountain lodges. Many were injured and unable or unwilling to risk descending 3,067-metre Mount Ontake on their own.

Rescue workers were also trying to reach the area on foot Sunday morning, Hanamura said.

Lodge managers are familiar with first aid procedures and were communicating with rescue officials in town, he said.

'Totally covered with ash'

With a sound likened to thunder, the volcano erupted on a clear autumn day, spewing large white plumes of gas and ash high into the sky and blanketing the surrounding area in ash.

Smaller eruptions continued overnight. 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.

One witness told NHK that the eruption started with large booms.

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

Many of those who made it down emerged with clothes and backpacks covered in ash. They reported being engulfed in total darkness for several minutes.

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 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 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 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.