The men ran into the shelter when the volcano spewed a hot cloud of gas and ash that reached seven kilometers (four miles) down its southeastern slopes on Wednesday, sparking panic among residents.

"Rescuers found the corpse of the second person near the bathroom while the corpse of the other was found near the bunker's door," said Lieutenant Colonel Mursal, who headed a rescue team that got through to the bunker.

He said rescuers concluded that both men were killed by the heat of the cloud, estimated at around 400 degrees Celsius (750 degrees Fahrenheit).

The pair were helping people evacuate their village when the heat cloud and molten lava struck. They were the first human casualties since Merapi began showing increased activity in April.

The condition of the bodies was similar to having been "baked in an oven," a rescue worker who went inside the bunker using heat-protected clothing told an AFP journalist at the site.

Merapi has been emitting deadly heat clouds that can reach temperatures of up to 500 degrees Celsius.

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

On Friday the volcano emitted three heat clouds five kilometers down its southestern slopes, near the site of the bunker where the men were killed, said Triyani of the volcanology office in nearby Yogyakarta city.

She said the men had had ample time to flee.

"They should have fled when there was a sign that the cloud was coming. We don't know the condition of the bunker, whether it's up to standards," she told AFP.

One man who witnessed Wednesday's eruption and was a friend of one of the victims said the two could have survived if they had fled immediately.

"His motorcycle was on stand-by. He shouldn't have entered the bunker," the man told ElShinta radio.

Sunartono, a local relief official, said the bunker had been designed to protect people from passing heat clouds and that the men had died because the bunker was buried under 2.5 meters (yards) of thick hot molten rock.

"Because the bunker was covered by extremely hot materials, the temperature inside was also very hot," he was quoted as saying by the Detikcom news website.

Wednesday's eruption also razed a camping ground popular among Indonesian mountaineers and nearby food stalls, said local official Mulyanto.

"Trees were burned and food stalls were destroyed by volcanic material. So far no houses were destroyed," he told AFP.

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

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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