Seven more bodies have been found by rescuers combing the peak of a Japanese volcano that erupted at the weekend, taking the death toll to at least 43.

The seven are in addition to the 36 already known to have died when Mount Ontake burst angrily to life during the busy hiking season.

"Rescuers found seven more people in cardiac arrest near the mountain top, and are now carrying them down," an official in Otaki village, where one mountain trail begins, said.

Only doctors can declare someone officially dead, so first responders typically report that someone's heart has stopped and they are not breathing.

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The find comes after media reports suggested as many as 20 people remain unaccounted for, with an area of the volcano still out of bounds to rescuers as it continues to billow toxic gas and steam.

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Some of the around 1000 troops, police and firefighters combing the volcano succeeded in bringing down 14 of the bodies that were discovered on Sunday, with 10 more still there.

An official at Nagano prefecture's crisis management office said helicopters had been used to ferry the dead from the mountain, where the pock-marked lunarscape bears witness to the huge volume of ask and rocks that were flung from the volcano.

"We believe there are more people still missing, but we don't know how many they are," he added.

Hiking is an incredibly popular pastime in Japan, with mountain trails promoted by tourism officials who ask walkers to sign in when they begin their trek and sign out again when they finish.

But a local tourism association told the Asahi Shimbun "usually only 10 to 20 per cent of hikers register their names with authorities before entering the mountains in high season".

The report said 327 hikers had registered to be on Mount Ontake at the time of the eruption.

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Rescuers are hoping that many of those that cannot be contacted simply forgot to let mountain managers know they were safe.

However, there exists the grisly possibility that many more perished.

"We don't know if there are people buried deep down under accumulated ash," a senior police official told the Asahi.

The local fire department said 71 people are missing, while the Nagano prefectural police has received at least 240 reports of people whose whereabouts are unknown, it said.

Both organisations cautioned that some of these reports would likely have nothing to do with the disaster, which happened without warning during a busy hiking weekend.

Volcanic tremors have been detected constantly since Saturday's eruption, with underground water boiling into steam and breaking or moving rocks, a volcanologist at the meteorological agency said.

The agency warned on Wednesday the eruption was still under way, and noted that smoke had been seen issuing from the volcano as of 9.00am.

-AAP