Mount Rokatenda, on the island of Palue, sent huge clouds of red-hot ash and rocks two kilometres (1.2 miles) into the air, said vulcanology centre head Surono. (Agencies)
"There are six victims who were engulfed in hot lava," Johanes Berchmans, head of the disaster management agency in Sikka district, said.
"They were swept away by hot lava when they were asleep on Punge beach,” Berchmans said. Three adults aged between 58 and 69 were found dead while three children aged between five and eight were also killed, though their bodies had not yet been recovered, he said.
Surono, who like many Indonesians goes by one name, said that rescue efforts were difficult "because the area is still very hot". Rokatenda has been showing signs of increased volcanic activity since October, with authorities banning people from any activities within three kilometres of its crater.
Some 2,000 people had previously been evacuated from the island due to the volcanic activity leaving around 8,000 before Sunday's eruption, said Berchmans. "Tomorrow our staff will go from area to area to pick up anyone willing to leave the island," he said.
The volcano began erupting at 04:27 am (2027 GMT on Saturday) and it continued for nearly four hours, said Surono.
Palue is part of Sikka district, which is part of East Nusa Tenggara province. It is just north of Flores island in the sprawling Indonesian archipelago which is made up of more than 17,000 islands.
Indonesia has dozens of active volcanoes and straddles major tectonic fault lines known as the "Ring of Fire" between the Pacific and Indian oceans. The country's most active volcano, Mount Merapi in central Java, killed more than 350 people in a series of violent eruptions in 2010.
Mount Rokatenda, on the island of Palue, sent huge clouds of red-hot ash and rocks two kilometres (1.2 miles) into the air, said vulcanology centre head Surono.