2 villagers trapped in underground shelter on Indonesia's Mount Merapi

MOUNT MERAPI, Indonesia -- Soldiers wearing heat-retardant clothing dug through volcanic debris Thursday trying to reach two men trapped in an underground emergency shelter on Indonesia's most volatile volcano.

The fierce heat melted the troops' shovels and the tires on a mechanical digger brought in to plow through more than six feet of debris covering the bunker, which was built for protection from volcanic eruptions, said Maj. Sunarso, who goes by one name.

The two men have been trapped since Wednesday when Merapi sent massive clouds of searing gas and rock fragments rolling far down its slopes. Rescuers made contact with one of the men late Wednesday, but their phones were no longer working and it was unclear if they were still alive.

By nightfall, the soldiers had dug away the debris surrounding the door, but found it blocked by a large hot boulder, said senior disaster official Heka, who also goes by a single name. The operation was suspended until this morning.

Merapi continued to spew out scorching gas clouds and rock fragments Thursday, with one black plume in the mid-afternoon covering a large swathe of the mountain. No injuries or fresh property damage were reported.

Scientist Antonius Ratdomopurbo said the resurgence in activity was likely caused by the collapse of a section of the volcano's lava dome, which has been growing in recent weeks as lava emerges from its core.

The two trapped men, believed to be emergency workers helping evacuate villagers from the mountain, were inside the shelter under volcanic debris as hot as 572 degrees Fahrenheit, officials said.

The bunkers, several of which dot the slopes of Merapi, are typically equipped with water and food and emergency supplies of oxygen.

"God willing, they can survive for three days," senior disaster official Heka, who also uses a single name.

The main dangers at Merapi are fast-moving bursts of blistering gases and rock fragments called pyroclastic flow. Experts say a massive vertical eruption threatening people many miles away will not occur.