An Indonesian volcano erupted on Tuesday, prompting terrified villagers to flee and join the thousands already evacuated from its slopes.

The peak of Mount Merapi was clouded with smoke, making it impossible to see if lava was spewing out, but ash was raining down on frightened residents.

Authorities had been trying since Monday to evacuate more than 11,000 villagers living on the volcano's slopes, after the alert status was raised to the highest level.

Adi Mulyanto, an emergency ward doctor at the Panti Nugroho Hospital in Sleman, told Reuters that at least six people had been badly burned by hot air bursting out of the volcano.

"Three of those people have been rushed to hospital with burns to more than 80 percent of their bodies," he said.   

A Reuters cameraman on the slope of the mountain said he saw burns victims being carried away.   

Merapi, on Java island, erupted on Tuesday evening, a day after the alert status was raised to the highest level.   

In a 1994 eruption, after the volcano's lava dome collapsed, 70 people were killed. The volcano killed 1,300 people in 1930.

Last month, another Indonesian volcano, Mount Sinabung, on Sumatra island to the west, erupted after laying dormant for 400 years, forcing a mass evacuation.-Reuters