May 15 (Bloomberg) -- Indonesia's Mount Merapi, the nation's most active volcano, is spewing lava and superheated gas, two days after authorities raised their threat assessment of an eruption to the highest level.

``Heat clouds have reached 4.2 kilometers (2.6 miles) down the slope so far, and it could expand up to 10 kilometers,'' Ratdomo Purbo, head of the Volcano Development and Research Center in Yogyakarta city, said in a briefing broadcast on ElShinta radio news. In 1994, when Merapi last erupted, heat clouds spread as far away as 6 kilometers with temperatures reaching 600 degrees Celsius, he said.

Purbo said the lava dome, created by several individual lava flows, is intact. Lava dome collapses can set off pyroclastic flows, or the superheated flow of molten rock, ash and gases.

Scientists said the process would build up gradually and that the full eruption had yet to take place. One of the concerns is that the lava dome that has been forming at its peak over the past two weeks will collapse.

Purbo said volcanic activities will continue to intensify in the night time because of the moon's gravitational pull.

The government is evacuating more than 3,000 people in eight villages living near the volcano, said Edy Susanto, head of the public welfare office in Magelang city, west of Merapi.

Pyroclastic flows could spread to a radius of between 8 kilometers and 12 kilometers in coming days, Subandrio, head of the Mount Merapi observatory at the center, said on May 13.

Heat Clouds

As of noon local time, heat clouds had descended from the mountain 44 times, Purbo said. Mount Merapi is located near Yogyakarta city, about 400 kilometers southeast of the capital, Jakarta, on Indonesia's most densely populated island of Java.

Many villagers returned to their homes to check on their animals, Susanto said. He described this as being ``too dangerous.''

According to the Darwin Volcanic Ash Advisory Center, which advises airline companies about volcanic debris, no ash from Merapi was visible on satellite images.

The Smithsonian Institute's Global Volcanism Program defines an eruption as explosive ejection of fragmental material, the effusion of liquid lava, or both. While lava is spewing from Merapi, there hasn't been a violent ejection of volcanic debris.

Evacuation Plan

About 17,000 villagers living on the slopes of Merapi need to be evacuated, Vice President Jusuf Kalla said on May 11. The volcano rained about 5 millimeters (0.2 inch) of ash on nearby villages, Antara news agency reported, citing local residents.

Aid group Oxfam is distributing kits containing binoculars, flashlights, kerosene lamps and loudspeakers to as many as 30 villages living close to the volcano, the U.K.-based group said on its Web site.

Merapi last erupted in 1994, killing 64 people and displacing 6,000. Indonesia, the world's largest archipelago, has 129 active volcanoes. The nation's 18,000 islands are prone to earthquakes because the country sits along the Pacific Ocean's so called ``Ring of Fire'' zone of active volcanoes and tectonic faults.

Merapi is one of 16 volcanoes, known as Decade Volcanoes, which are studied by the International Association of Volcanology and Chemistry of the Earth's Interior because of frequent eruptions near populated areas, in a bid to improve hazard awareness.

Pyroclastic Flows

The volcano is unique in ``terms of frequency of the pyroclastic flows,'' Toshitsugu Fujii, vice president of the association and a professor for earthquake research at the University of Tokyo, said on April 17.

Pyroclastic flows can reach speeds of more than 100 kilometers an hour. Between 1600 and 1982, volcanic eruptions have resulted in the deaths of more than 160,000 people in Indonesia, the most in any region, according to ``Volcanic Hazards: A Sourcebook on the Effects of Eruptions.''

Indonesia was the site of two of the world's biggest volcanic eruptions. Mount Tambora's eruption in 1815 was the world's biggest recorded eruption, while Krakatau in the Sunda Straits exploded in 1883. The resulting tsunami from Krakatau's eruption killed thousands of people.

To contact the reporter on this story: Karima Anjani in Jakarta at kanjani@bloomberg.net.

Last Updated: May 15, 2006 08:35 EDT