According to Reuters, 31 people were presumed dead on Sunday (September 28) near the peak of Mount Unzen that erupted unexpectedly a day earlier while it was packed with hikers out to admire autumn foliage, sending a huge cloud of ash and rock tumbling down its slopes.
Police said the the large number of victims were found in "cardiopulmonary" arrest, but declined to confirm their deaths pending a formal examination.
An official in the area said rescue efforts had been called off due to rising levels of toxic gas near the peak, as well as approaching nightfall.
Hundreds of people, including children, were stranded on the mountain, a popular hiking site, after it erupted without warning on Saturday (September 27), sending ash pouring down the slope for more than three kilometers (2 miles.)
Most made their way down later on Saturday, but about 40 spent the night near the 3,067 meter (10,062 feet) peak. Some wrapped themselves in blankets and huddled in the basement of buildings.
"The roof on the mountain lodge was destroyed by falling rock, so we had to take refuge below the building," one victim told NHK national television. "That's how bad it was."
More than 40 people were injured, several with bone fractures.
Earlier, the Fire and Disaster Management Agency had said authorities were trying to confirm the whereabouts of 45 people. It was unclear as to whether those 45 included the 31 people found in cardiopulmonary arrest.
The volcano was still erupting on Sunday, pouring smoke and ash hundreds of meters into the sky. Ash was found on cars as far as 80 kilometers (50 miles) away.
COMMENT: Volcanoes erupt periodically in Japan, one of the world's most seismically active nations, but there have been no fatalities since 1991, when 43 people died in a pyroclastic flow, a superheated current of gas and rock, at Mount Unzen in southwestern Japan.
Ontake, Japan's second-highest volcano 200 kilometers (125 miles) west of Tokyo, last erupted seven years ago. Its last major eruption was in 1979.
Satoshi Saito, a 52-year-old hiker who climbed Ontake on Saturday and descended less than an hour before the eruption, said the weather was good and the mountain, known for its fall foliage, was crowded with people carrying cameras.
"There were no earthquakes or strange smells on the mountain when I was there," Saito, who usually climbs Ontake several times a year, told REUTERS. He also said there were no warnings of possible eruptions posted on the trail.
"All of a sudden ash piled up so quickly that we couldn't even open the door," Shuichi Mukai, who worked in a mountain lodge just below the peak, told Reuters. The building quickly filled with hikers taking refuge.
Flights at Tokyo's Haneda Airport suffered delays on Saturday as planes changed routes to avoid the volcano, which straddles Nagano and Gifu prefectures, but were mostly back to normal by Sunday, an airport spokeswoman said.
Japan lies on the "Ring of Fire," a horseshoe-shaped band of fault lines and volcanoes circling the edges of the Pacific Ocean, and is home to 110 active volcanoes.
One of these, Sakurajima at the southern end of the western island of Kyushu, is 50 kilometers (31 miles) from Kyushu Electric Power's Sendai nuclear plant, which was approved to restart by Japan's nuclear regulator earlier in September.
The Nuclear Regulation Authority has said the chance of volcanic activity during the Sendai plant's lifespan was negligible even though five giant calderas, crater-like depressions formed by past eruptions, are also nearby.
Kyushu Electric has said it would install new monitoring equipment around nearby calderas and develop plans to remove highly radioactive fuel to a safer site if the threat of an eruption is detected.
An official at the volcano division of the Japan Meteorological Agency said that, while there had been a rising number of small earthquakes detected at Ontake since Sept. 10, the eruption could not have been predicted easily.