(Reuters) - Search and recovery efforts for at least two dozen victims of Japan's worst volcanic eruption in decades were called off on Tuesday due to worries about rising volcanic activity, including the chance of another steam explosion.

Hundreds of military searchers had been preparing to enter Mount Ontake by foot and helicopter to resume recovery of at least 24 people caught in a deadly rain of ash and stone after the peak erupted without warning on Saturday when it was crowded with hikers, including children.

Twelve bodies have been recovered from the 3,067-metre (10,062 feet) peak but at least 36 are feared to have died, with recovery hampered by high levels of toxic gas and ash piled hip-high in places on the still-erupting mountain. At least 69 people have been injured, 30 of them seriously.

"I just want to know something soon," said Kiyokazu Tokoro to Japanese media. His 26-year-old son was on the mountain with his girlfriend and has yet to be found.

Increasingly strong volcanic tremors on Tuesday morning have raised fears that the peak could spew out more rock or even be heading towards another steam explosion, an official at Japan's Meteorological Agency, which monitors volcanoes, told Reuters.

"The strength of the tremors increased late last night, diminished and then rose again early this morning. There's the chance things could get even worse, so caution is needed," said Yasuhide Hasegawa, at the agency's Volcano Division, adding the chance of an explosion like Saturday's was small but could not be rule out.

"This points to possibly increasing pressure due to steam inside the volcano, and if it exploded rocks could be thrown around, endangering rescuers," he added.

The weekend explosion may have propelled rocks so violently they could have reached the speed of an airplane, said Kazuaki Ito, a volcanologist who surveyed Ontake after it erupted in 1979, its first eruption in recorded history.

"It is hard to know how the victims died. They may have been struck by rocks or inhaled ash," he told NTV.

More than 800 rescuers were standing by and would head up into the mountain later in the day if things calmed down, said a firefighter waiting at rescue headquarters in the foothills as the peak spewed smoke and ash some 400 meters into the sky.

Hazards on the peak, transformed into an eerie moonscape by gray ash, are already numerous, ranging from ash flung up by helicopter blades to loose stones that make footing dangerous and toxic gas.

Most of the victims appear to have been found near a shrine at the narrow, rocky top of the peak, Japanese media said. They may include 11-year-old Akari Nagayama, who reached the summit earlier than the rest of a group that included her mother.

Japan is one of the world's most seismically active nations. In 1991, 43 people died in a pyroclastic flow, a superheated current of gas and rock, at Mount Unzen in the southwest.

Ontake, Japan's second-highest active volcano, last had a minor eruption seven years ago.