Villagers living in the shadow of Merapi have been asked to move to shelters as a precautionary measure [EPA]

Indonesia's Mount Merapi has begun to erupt, spewing searing clouds of volcanic ash and rocks into the sky, the country's chief vvolcanologist said.

Surono, a government volcanologist, said the mountain, which had rumbled for hours, started to erupt just before dusk on Tuesday, a day after the agency raised the alert status to the highest level.

"We heard three explosions around 06:00 pm (1100GMT) spewing volcanic materials as high as 1.5 kilometres upward and heatclouds down on the slopes," Surono told the AFP news agency.

Indonesian scientists had warned of a pressure build-up beneath Mount Merapi's lava dome, as authorities evacuated tens of thousands of villagers to temporary shelters.

An avalanche of rocks spilled down Merapi's slopes before dawn on Tuesday and gusts of ash rose 50 metres into the air as the mountain continued to rumble.

Surono had said earlier that the greatest concern was pressure building behind a massive lava dome that has formed near the tip of the crater.

"The energy is building up. ... We hope it will release slowly," he said.

"Otherwise we're looking at a potentially huge eruption, bigger than anything we've seen in years."

Temporary shelters were set up after officials designated an area 10km around the country's most active volcano a danger zone.

More than 50,000 people living in the shadows of the volcano, located north of the Javanese cultural capital, Jogjakarta, are now facing evacuation.

Urgent evacuation

Widi Sutikno, a field co-ordinator in the Sleman district on the southern slopes of the 2,914-metre mountain, said about 3,700 people already moved to makeshift camps.

"We have evacuated many women, pregnant women, sick people, elderly people and children," Sutikno said.

"We let some people return to their fields for their daily activity. But they need to go back to the camps and not their houses."

Before the eruption, Step Vassen, Al Jazeera's correspondent in Jakarta, the capital, said: "Most residents are still in their villages. That's because this happens a lot. This volcano has erupted many times in the last couple of years and people have been evacuated only to have to return home.

"So people are quite reluctant to leave.

"The most dangerous part of this volcano is very hot clouds released by the mountain that can reach up to 500 degrees Celsius and they have a high speed of up to 200 kilometres per hour. And these clouds have killed the most people in the last year."

Merapi last erupted in 2006, when it sent an avalanche of pyroclastic ash - hot gases and rock fragments - racing down the mountain which killed two people.

A similar eruption in 1994 killed 60 people, while 1,300 people died in an eruption in 1930.

There are more than 129 active volcanoes in Indonesia, which are spread across 17,500 islands and prone to eruptions and earthquakes due to its location within the so-called Ring of Fire - a series of fault lines stretching from the western Hemisphere through Japan and Southeast Asia.

Source: Al Jazeera and agencies