More than 30 hikers have been found in “cardiac arrest” near the peak of an erupting volcano in Japan, police say, using a term usually applied before doctors are able to certify death.

Rescue workers found the hikers near the summit of 3,067-metre Mount Ontake, which erupted about noon on Saturday (local time).

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 “We have confirmed that more than 30 individuals in cardiac arrest have been found near the summit,” a Nagano prefecture police spokesman told the AFP news agency without elaborating further.

Rescuers were planning to bring four people down the mountain by later tonight, Jiji Press said citing police sources.

In Japan, the term “cardiac arrest” is used by emergency services for people without vital signs.

More than 500 Japanese military and police set out earlier today to search the peak of the volcano which is popular with hikers.

About 150 people, including children, were stranded on the peak after it erupted without warning, sending ash pouring down the slope for more than three kilometres.

Local officials believe around 45 hikers sheltered overnight in cabins on the popular mountain, although details remained unclear.

The mountain is popular among hikers particularly in late September as autumn leaves change colour.

Mount Ontake erupts. Photo: AAP

Most people made their way down that evening however about 30 remained stranded on Sunday morning.

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

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

A helicopter from Japan’s Self Defence Force. Picture: Getty

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.”

Rescue team workers rush to Mt. Ontake. Picture: Getty

The mountain, some 200 kilometres west of Tokyo, is a popular site to view autumn foliage, currently at its best.

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 Mt Unzen in the country’s south-west.

Rescue workers patrol near the peak of Mount Ontake. Photo: AAP