"Because of its position smack in the middle (of the peak), this means that it has the opportunity to expand," geologist Ratdomo Purbo said, referring to the lava dome being formed by magma flowing out of a fissure at Merapi's peak.

The new dome has been forming on Merapi for nearly two weeks, and scientists have said it indicates that an impending eruption would involve an outflow of lava and deadly heat clouds rather than a massive explosion.

Purbo, speaking from the volcanology office in Yogyakarta, 30 kilometers (18.5 miles) south of Merapi, also said small rocks were being hurled out of the top of the crater.

The office, he said, still saw no reason to raise the alert level on Merapi, which towers over a fertile Central Java plain and provides livelihoods for thousands of people living around its slopes.

The 2,914-meter (9,560-foot) Merapi was put on "standby" alert status more than three weeks ago. The status was one rung below that requiring the mandatory evacuation of residents on the volcano's slope.

More than 5,000 people have fled their homes around the volcano so far but many return to work near their homes during daylight hours.

In its last large eruption in 1994, heat clouds known locally as "shaggy goats" careened down the volcano at more than 100 kilometres (60 miles) per hour, reaching temperatures of 600 degrees Celsius (1,100 degrees Fahrenheit).

The clouds killed 66 people.

Merapi's most deadly eruption occurred in 1930, when 1,369 people were killed.

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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