Mount Lewotobi Laki-Laki, located on the popular tourist island of Flores, spewed ash and lava that peppered the surrounding area with fireballs, forcing authorities to evacuate several villages.

"Six fatalities have been confirmed," Abdul Muhari, spokesman of the country's disaster management agency, 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.

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

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

The country's volcanology agency 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.

- Ash rain -

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

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.