Rescuers have fought throughout the day with melting spades and two mechanical diggers trying to reach two men trapped on the side of Mount Merapi, Indonesia's most active volcano.

Surrounded by lava and blocked in by boulders as hot as 300C, the men now face spending a second night on the mountain.

The men have not been heard from since yesterday, when they hid in an emergency shelter designed to withstand an eruption. As night fell, the rescue attempt was called off until first light tomorrow.

Floating hot clouds of gas frequently interrupted today's digging, which was carried out by around 50 men in the district of Cangkringan, on the southwestern slope of the volcano. This evening, a hot boulder was found blocking the entrance to the shelter, which has enough oxygen, food and water to last the men for several days.

"The search was halted because the volcano was emitting more hot clouds and ash," Suryadi, a local rescue official told Agence France Presse. "Workers were afraid and left the area. Maybe they will return when the situation calms," he said.

Asked if he knew whether the men were still alive, Suryadi replied: "We don’t know if they’re alive, we have to dig."

The two men are believed to be rescue workers who were helping evacuate villagers from the slopes of Mount Merapi yesterday when a cloud of hot gases rushed 7km (4 miles) down the volcano. It was one of series of eruptions that halted the gradual return of thousands of people who have fled the mountain in recent weeks.

Mount Merapi has flared and fallen silent, threatening to explode, for a month. Around 11,000 people have sought shelter in camps pitched outside the range of an eruption but they have been tempted back to their homes, and then sent fleeing again, by lava flows and the appearance of dangerous ash clouds.

Last Friday, a lava dome created by the recent activity collapsed in the volcano's main crater, leading some geologists to speculate that the mountain would be calmed. But yesterday's eruptions, which covered houses to the south-east of Mount Merapi with 5mm of ash, prompted Indonesia's Vulcanology Office to put the region once again on the highest alert.

Mount Merapi is one of 70 active volcanoes in Indonesia. In 1994, 60 people were killed by an eruption. In 1930, more than 1,300 people lost their lives in the worst eruption of recent times.

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