Police confirm that more than 30 individuals in cardiac arrest were found after mountain erupted on Saturday without warning

At least 30 people are believed to have died near the peak of a volcano in central Japan that erupted without warning on Saturday, trapping scores of amateur climbers and covering a wide area with thick ash.

Police said rescuers had discovered more than 30 people suffering from heart and lung failure; official confirmation that the victims are dead won’t come until doctors have examined the bodies.

“We have confirmed that more than 30 individuals in cardiac arrest have been found near the summit,” a Nagano prefecture police spokesman told Agence France-Presse.

Mount Ontake, which straddles Nagano and Gifu prefectures about 130 miles west of Tokyo, erupted shortly before noon on Saturday, sending thick plumes of smoke and rocks into the air.

About 250 people were near the 3,067-metre peak when the eruption occurred. Most were able to make their way down the mountain to safety on Saturday, reports said.

Japanese rescue workers carry an injured person from a mountain lodge on Mt Ontake. Photograph: Kyodo/Reuters

Before the bodies were discovered on Sunday, the fire and disaster management agency had said 45 people were unaccounted for. After calling off their search overnight on Saturday amid fears of fresh eruptions and the buildup of toxic gas, rescue workers were continuing to reach people on Sunday, including several who were injured, who had taken refuge overnight in mountain lodges.

Seven people were taken off the mountain on Sunday morning in a rescue operation involving helicopters and more than 500 troops, police officers and firefighters.

Twenty-six climbers who had stayed in a lodge on the Gifu side made their way down early on Sunday and reached the foot of the mountain before noon. Two were seriously hurt and six had minor injuries, Kyodo news agency said.

Witnesses said they initially mistook the eruption for thunder. “For a while I heard thunder pounding a number of times,” said Shinichi Shimohara, who works at a shrine at the foot of the mountain. “Soon after, some climbers started descending. They were all covered with ash, completely white. I thought to myself, this must be really serious.”

The mountain, which last had a major eruption in 1979, is popular with weekend hikers, particularly in September and October when the autumn leaves are at their best.

Cloud of volcanic ash rises from Mount Ontake. About 250 people were near the 3,067-metre peak when the eruption occurred. Photograph: Kazuhiro Nogi/AFP/Getty Images

Officials said the volcano had shown no indication that it was about to erupt with such force. An official at the volcano division of the meteorological agency said that although there had been a rising number of small earthquakes detected at Ontake since 10 September, 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 said. “With only the earthquakes, we couldn’t really say this would lead to an eruption.”

But David Rothery, a professor of planetary sciences at the Open University, said he was surprised that Ontake had erupted without warning.

“This eruption of Ontake was not especially powerful, but distributed several centimetres of ash across areas close to the summit. It seems similar to its 1979 eruption, which is the only other eruption of this volcano in historic times,” Rothery said.

“I am surprised that it erupted without warning because so far as I know there are seismometers on Ontake that are supposed to detect signs of internal magma movement. ‘Sneak’ eruptions lacking in precursor signs are not unknown, but analysis of the records may reveal data that, with hindsight, should have been read as a warning sign. This is how we learn how to do it better next time.”

Video footage taken with mobile phones showed huge grey clouds engulfing climbers, who were forced to halt their ascent as the area was plunged into darkness.

“All of a sudden ash piled up so quickly that we couldn’t even open the door,” said Shuichi Mukai, who works in a mountain hut just below the peak. The hut 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,” Mukai added.

Flights at Tokyo’s Haneda airport were delayed as planes changed routes to avoid the peak, but were mostly back to normal by Sunday, an airport spokeswoman said.

Volcanoes erupt periodically in Japan, one of the most seismically active countries in the world, but there have been no deaths since 1991, when 43 people died in a pyroclastic flow – a superheated current of gas and rock – at Mount Unzen in south-west Japan.

The meteorological agency advised climbers to stay away from Ontake and forecast further eruptions, warning that volcanic debris may settle within 2.5 miles of the peak. The agency called on local residents to remain alert, as the force of the eruptions could shatter windows miles away.