The dome has been expanding since last Wednesday behind another dome that was formed in 1997, said Dewi Sri from the vulcanology office in the ancient cultural city of Yogyakarta, 30 kilometers (18 miles) south of the volcano.

"It continues to grow and its volume has now reached into the hundreds of cubic meters... It is a sign that the magma pressure is increasing," she told AFP.

She said the mountain has significantly increased its activity, with a higher frequency of multi-phased earthquakes as well as those caused by lava fallout.

"All this indicates that magma pressure is building up and an eruption could follow," Sri said.

The alert status of the 2,914-meter (9,560-foot) volcano remained on "standby" however, as it has for more than two weeks, one level below that which would require a mandatory evacuation for more than 29,000 people.

Ratdomo Purbo, who heads the Vulcanology Research and Technology Development Office in Yogyakarta, was quoted by the Koran Tempo newspaper as saying that the dome had now grown some 10 meters (33 feet) high.

Purbo, who could not be reached for comment on Tuesday, was also quoted as saying that should the dome break or burst, it would spew lava accompanied by pyroclastic flows, or heat clouds.

In its last large eruption in 1994, heat clouds known locally as "shaggy goats" reached 600 degrees Celsius and speeds of over 100 kilometers per hour. They killed 66 people on the southern slope of the mountain.

Hundreds of residents have already been relocated to temporary shelters but many living on the volcano's slopes have refused to leave. Merapi, which has been rumbling intermittently over the past four years, looms above a plain in the south of Central Java province.

Indonesia sits on the Pacific "Ring of Fire" noted for its volcanic and seismic activity. The country has more than 100 active volcanoes.

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