"The 'beware' status was reimposed at close to 3:00 pm (0800 GMT) after flows of clouds descended on the southern slopes, reaching about six kilometers (four miles)," said Triyani, a scientist from the volcanology office in Yogyakarta, the main city south of Merapi.

The red alert had been downgraded Tuesday morning after the volcano's activities appeared to be slowing.

But on Wednesday at around midday, it sent searing clouds - indicative of a typical eruption at Indonesia's second most active volcano - down a gully running down the southern slope for about 30 minutes, Triyani said.

Almost three hours later, the volcano spewed a continuous cascade of clouds that reached up to six kilometers down the same gully - one kilometre further than earlier.

"The first thing is that the clouds have now reached six kilometers (down the slopes)," the head of the Merapi monitoring section at the vulcanology office in Yogyakarta, Subandriyo, told ElShinta radio when asked about the reasons behind the reinstatement of the alert.

"The other important thing is that clouds are descending continuously."

Subandriyo said the clouds had left deposits behind, making the natural channels they were following shallower and allowing them to reach further down the slopes.

He said authorities had been asked to re-evacuate residents from the earlier danger zone and all those living within a radius of seven kilometers from the peak.

"We began re-evacuating people again at around 4:00 pm this afternoon," said Warsito, an official from the district disaster management center in Sleman, one of three affected districts.

Agus Pujiwinarno, head of Kepuharjo village in Sleman, said he had received unverified reports that two buildings had been burned by the clouds. If confirmed, it would be the first damage to property caused by Merapi.

"I have received reports, which although I have not yet verified appear to be true, saying that two buildings at the Bebeng volcano observation site have been burned by clouds," he said.

The buildings - an open hall and vulcanoloy post - were just over six kilometres from the peak, he said.

"But there were no victims, as everyone had already vacated the site."

ElShinta radio reported that panic gripped people at Srumbung in Magelang district on Merapi's western slopes, an area not declared under threat, when they saw the clouds being carried by the wind towards them.

Villagers elsewhere fled back to the safe shelters they had left just a few hours earlier, it reporter without giving figures. Scientists initially declared a top alert on Merapi on May 13.

Merapi has shown fluctuating volcanic activity since then but had notably declined since a lava dome that was forming at its peak partially collapsed last Friday.

Its deadliest eruption was in 1930 when more than 1,300 people were killed.

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