MOUNT MERAPI, Indonesia -- Indonesia's most volatile volcano groaned and rumbled Tuesday and scientists warned that pressure building beneath its lava dome could trigger one of the most powerful blasts in years. Thousands of residents living on its slopes were moved to temporary camps.

An avalanche of rocks spilled down Mount Merapi's trembling slopes before dawn and gusts of hot ash shot 150 feet (50 meters) into the air.

The greatest concern was pressure building behind a massive lava dome that has formed near the tip of the crater, government volcanologist Surono told reporters.

"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."

The alert level for the 9,737-foot (2,968-metre) mountain has been raised to its highest level.

In 2006, an avalanche of blistering gases and rock fragments raced down the volcano and killed two people. A similar eruption in 1994 killed 60 people, and 1,300 people died in a 1930 blast.

While bracing for Mount Merapi to explode, officials were also trying to cope with damage from a 7.7-magnitude earthquake late Monday that caused a tsunami off western Indonesia, leaving scores of villagers dead or missing.

Indonesia, an island archipelago of 237 million people, is prone to earthquakes, tsunamis and volcanic activity due to its location on the so-called Pacific Ring of Fire -- a series of fault lines stretching from the Western Hemisphere through Japan and Southeast Asia.

Sri Purnomo, the head of Sleman district on Java island, where Mount Merapi is located, said officials were warning some 11,400 villagers living on the mountain's southern slope to prepare for "urgent evacuation."

By late Tuesday, more than 2,200 people, most of them elderly or children, had relocated to makeshift camps in Hargobinangun, a village 10 miles (17 kilometers) from the volcano's base, said local village chief Bejo Mulyo.

"I just have to follow orders to take shelter here for safety even though I'd rather like to stay at home," said Ponco Sumarto, 65, who arrived at one of the camps with her two grandchildren.

She said her children stayed at home to take care of their livestock and crops.

There are more than 129 active volcanoes to watch in Indonesia, which is spread across 17,500 islands.