Japanese rescuers Monday resumed a grim operation to recover more bodies after 31 people were believed killed in a volcanic eruption, as survivors told of seeing hikers die when tonnes of ash and rocks thundered from the sky.

More than 1,100 firefighters, police and troops returned to Mount Ontake in a bid to reach those they had to abandon on Sunday when soaring levels of poisonous gas made the operation too dangerous.

Smoke was still rising from the peak Monday, but helicopters had begun a search of the volcano, a popular destination for walkers during autumn.

Aerial footage showed rescue workers climbing slopes blanketed with thick, grey ash.

At least 31 people were found near the summit of 3,067-metre (10,121-foot) volcano, which erupted Saturday without warning, spewing ash, rocks and steam. Emergency workers who reached them said they were not breathing and had no pulse.

Four of them were brought down and later confirmed dead by doctors. They were all men, aged 23-61.

Heartbreaking stories have begun to emerge from survivors who made it down the mountain as rolling clouds of volcanic debris swept down its flanks, smothering everything in their path.

"Some people were buried in ash up to their knees and the two in front of me seemed to be dead," a woman hiker told the private Asahi network.

Another told how she had heard the last moments of a victim battered by a cascade of rocks.

"There was someone lying outside the hut after being hit in the back," she said. "He was saying 'It hurts, it hurts,' but after about half an hour he went quiet."

Seiichi Sakurai, who had been working at one of the huts around the top of the volcano, told public broadcaster NHK that he had tried his best to help people but could not save them all.

"Ash was constantly falling... Some people were buried alive but I could do nothing but tell (rescuers) about them over the radio," he said.

Another survivor told the Yomiuri newspaper that he had seen a boy shouting "It's hot" and "I can't breathe!" near the peak, before the ash clouds turned everything black and silent.

- 'It's over. I'm dying now' -

Rescue workers were trying to bring down at least 27 people still on the volcano who were believed killed, with six airlifted late morning, according to NHK. They were technically listed as in "cardiac arrest" -- a term applied before doctors can certify death.

About 40 people were hurt by flying rocks and inhaling poisonous fumes.

For anguished families, the wait for news was taking its toll.

A tearful father sobbed as he clutched a photograph of his son and the young man's girlfriend, who had not been heard from since the eruption.

An elderly woman told the Asahi network that her son had telephoned her just after gas, rocks and ash began spewing from the volcano.

"He told me it erupted... He said 'It's over. I'm dying now' and then the line was cut off," the woman said.

The meteorological agency forecast further eruptions, warning that volcanic debris may settle within four kilometres (2.5 miles) of the peak.

Japan's meteorological agency keeps a round-the-clock watch on 47 volcanos thought to be at risk of violent activity over the next century, including Mount Fuji, whose eruption could have a catastrophic effect on the country's economy.

But Toshitsugu Fujii, a vulcanologist at the agency, admitted accurate forecasting was very difficult.

Steam explosions such as those on Ontake often occur without warning, he said.

"People may say we failed to predict this (because there were earthquakes in September) but this is something that could not be helped, in a sense. That's the reality of the limit of our knowledge," he said Sunday.