Toxic gases and ash from a still-erupting Japanese volcano Monday forced rescue workers to call off their effort to bring down the rest of the victims, after earlier airlifting out eight more bodies by military helicopter.

The recovery work on the ash-blanketed peak was halted around 1:30 p.m., said Naofumi Miyairi, a spokesman for the Nagano prefecture police.

At least 31 people are believed to have died. Together with four victims flown down by helicopter on Sunday, 12 bodies have now been recovered, leaving 19 near the summit. Exactly how they died remains unclear, whether from gases, suffocating ash, falling rocks or other causes.

Scenes broadcast live on Japanese TV station TBS showed soldiers carrying yellow body bags one-by-one to a camouflage military helicopter that had landed in a relatively wide-open area of the now bleak landscape, its rotors still spinning.

The bodies were flown to a nearby athletic field, its green grass and surrounding forested hills contrasting with Mt. Ontake's ash-gray peak in the background, a reduced plume still emerging from its crater.

The bodies were then taken to a small, two-story wooden elementary school in the nearby town of Kiso, where they were being examined in the gymnasium.

Family members of the missing waited at a nearby municipal hall.

More than 200 soldiers and firefighters, including units with gas-detection equipment, were part of the search mission near the peak, said Katsunori Morimoto, an official in the village of Otaki.

The effort was halted because of an increase in toxic gas and ash as the volcano continued to spew fumes, he said. "It sounds like there is enormous ashfall up there."

The rescuers reported a strong smell of sulphur earlier this morning, Morimoto said.

Saturday's eruption was the first fatal one in modern times at 3,067-meter (10,062-foot) Mount Ontake, a popular climbing destination 210 kilometers (130 miles) west of Tokyo on the main Japanese island of Honshu. A similar eruption occurred in 1979, but no one died.

Japanese media reported that some of the bodies were found in a lodge near the summit and that others were buried in ash up to 50 centimeters (20 inches) deep. The four recovered Sunday were adult men, and the eight on Monday included both men and women.

Mount Ontake erupted shortly before noon at perhaps the worst possible time, with at least 250 people taking advantage of a beautiful fall Saturday to go for a hike. The blast spewed large white plumes of gas and ash high into the sky, blotted out the midday sun and blanketed the surrounding area in ash.

Hundreds were initially trapped on the slopes, though most made their way down by Saturday night.

About 40 people who were stranded overnight came down on Sunday. Many were injured, and some had to be rescued by helicopters or carried down on stretchers. By nightfall, all the injured had been brought down, officials said.

Japan's Fire and Disaster Management Agency tallied 40 injured people, three seriously, and said it was trying to update the number still missing.

Survivors told Japanese media that they were pelted by rocks. One man said he and others went into the basement of a lodge, fearing that the rocks would penetrate the roof. He covered himself with a futon, a thin Japanese mattress, for protection.

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Associated Press writers Mari Yamaguchi and Ken Moritsugu in Tokyo contributed to this report.