Mount Lewotobi Laki-Laki, a 1,703-metre (5,587-feet) twin volcano located on the popular tourist island of Flores, first erupted shortly before midnight, forcing authorities to evacuate several villages.

"Six fatalities have been confirmed," Abdul Muhari, spokesman of the country's disaster mitigation agency (BNPB), told Kompas TV.

Footage received by AFP showed villages near the volcano covered by thick ash, with some areas on fire.

An AFP journalist near the volcano said five villages evacuated, affecting thousands of residents.

Some wooden houses caught fire, and the ground was pockmarked with holes caused by flying molten rocks.

The crater erupted just before midnight and then again at 1:27 am (1727 GMT Sunday) and 2:48 am, the country's volcanology agency said.

It raised the alert level to the highest and told locals and tourists not to carry out activities within a seven-kilometre (4.3-mile) radius of the crater.

"There has been a significant increase in volcanic activity on Mount Lewotobi Laki-laki," it said in a press release Monday.

It released images that showed the roofs of houses collapsed after they were hit by volcanic rocks, and locals sheltering in communal buildings.

- Ash rain -

The volcanology agency warned there was a potential for rain-induced lava floods and told locals to wear masks to avoid the effects of volcanic ash.

There were a series of eruptions at the volcano last week, the biggest on Thursday, sending a column of ash 2,000 metres (6,500 feet) into the sky.

The mountain had several major eruptions in January, prompting authorities at the time to raise the alert status to the highest level and evacuate at least 2,000 residents.

Indonesia, a vast archipelago nation, experiences frequent eruptions due to its position on the Pacific "Ring of Fire", an area of intense volcanic and seismic activity.

In December last year, an eruption at one of the country's most active volcanoes, Mount Marapi in West Sumatra, killed at least 24 climbers, most of them university students.

And in May, more than 60 people died after heavy rains washed volcanic material from Marapi into residential areas, sweeping away homes.

That month Mount Ruang in North Sulawesi province erupted more than half a dozen times, forcing thousands of residents of nearby islands to evacuate.