The pair ran into the emergency shelter when the volcano emitted a heat cloud that reached seven kilometers (four miles) down its southeastern slopes on Wednesday, sparking panic among residents.

"The search was halted because the volcano was emitting more hot clouds and ash," Suryadi, a local official from Cangkringan district where the bunker is located, told AFP.

"Workers were afraid and left the area. Maybe they will return when the situation calms," he said.

Suryadi told AFP earlier in the day that workers had dug one meter into the debris covering the slope. He said the bunker was equipped with oxygen but electricity had been cut because of the clouds. Asked whether they were believed to be alive he said: "We don't know if they're alive, we have to dig."

Local village chief Heru Sutopo said about 50 rescue workers, including police and military, were digging at the site in Kaliadem village using a backhoe and shovels.

"It is not yet finished. We hope there won't be any victims but we have no idea whether or not they are alive. We hope that they are," Sutopo told AFP.

Mulyono, a local rescue agency official, said clouds had come close to the bunker at around 2:30 pm.

An official at the vulcanology office in Yogyakarta, the main city just south of Merapi, said that up until midday, 15 clouds had been emitted, reaching as far as 3.5 kilometres down the charred slopes.

Wednesday's large emission of cloud forced scientists to place Merapi back on its highest alert, meaning they believe an eruption is imminent, a day after they had downgraded it.

About 15,000 villagers sheltering in makeshift camps in safe areas had just begun returning home Wednesday when the clouds appeared. The volcano had been on red alert since May 13.

In the western district of Magelang however, which was previously seen as being the most at threat, fewer than 500 people remained in shelters by late Thursday, said Indarto, an evacuation agency official.

"We consider they are now safe to return to their homes," he said.

Some 11,000 people had been sheltering there before the alert was downgraded Tuesday.

Rescue agency official Mulyono said in the district of southeastern Sleman -- which lies in the current direction the clouds are taking -- more than 4,800 residents were sheltered in three barracks on Thursday.

The vulcanology office said earlier that the clouds on Wednesday had covered houses in the villages to the southeast which are closest to the peak with about five millimetres of ash.

Merapi has shown fluctuating volcanic activity since mid-May but appeared to stabilise after a lava dome that had been forming at its peak partially collapsed last Friday.

Merapi's deadliest eruption was in 1930 when more than 1,300 people were killed.

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