More than 30 people were feared dead on Sunday near the peak of the Japanese volcano that erupted a day earlier, sending a huge cloud of ash and rock tumbling down its slopes, while packed with hikers.

Police said they were found in a state of "cardiopulmonary" arrest, but declined to confirm their deaths pending a formal examination, as per Japanese custom.

Hundreds of people, including children, were stranded on Mount Ontake after it erupted without warning on Saturday, sending ash pouring down the slope for more than 3 km.

Most made their way down that evening but at least 30 spent the night near the 3,067 metre (10,062 feet) peak.

More than 40 people were injured, several with broken bones, and authorities were trying to confirm the whereabouts of 45 people, the Fire and Disaster Management Agency said earlier.

The volcano was still erupting on Sunday, pouring smoke and ash hundreds of metres into the sky. Ash was found on cars as far as 80 km (50 miles) away.

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.

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 bearing 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, adding that there were no warnings of possible eruptions posted on the trail.

"But a man who runs a hotel near the mountain told me that the number of small earthquakes had risen these past two months, and everyone thought it was weird."

Video footage on the internet showed huge grey clouds boiling towards climbers at the peak and people scrambling to descend as blackness enveloped them.

Footage on NHK national television showed windows in a mountain lodge darkening and people screaming as heavy objects pelted the roof.

"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.

"We were really packed in here, maybe 150 people. There were some children crying, but most people were calm. We waited there in hard hats until they told us it was safe to come down."

The mountain, some 200 km (125 miles) west of Tokyo, is a popular site to view autumn foliage.

Flights at Tokyo's Haneda airport suffered delays as planes changed routes to avoid the peak, which straddles Nagano and Gifu prefectures, but were mostly back to normal by Sunday, an airport spokeswoman said.

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.

"There were no other signs of an imminent eruption, such as earth movements or changes on the mountain's surface," the official told Reuters. "With only the earthquakes, we couldn't really say this would lead to an eruption." - Reuters