A volcano in western Indonesia that has been spewing clouds of searing gas high into the air has let out a powerful new burst, officials said.

On Wednesday, authorities were closely monitoring Mount Sinabung on Sumatra, one of Indonesia's main islands, after placing in at the highest alert level last week.

Hot ash tumbled down Sinabung's slopes up to 2.5km from its smouldering mouth, government volcanologist Surono told the Associated Press news agency.

He urged villagers to stay out of the main danger zone, which stretches 7km to the southeast of the crater.

No injuries were reported.

"The growing size of the lava dome is very unstable," said Surono, who goes by one name, adding that smouldering rocks mixed with hot gases may tumble down at any time.

The volcano in North Sumatra province has been shooting smoke and ash more than 500m into the air since Monday.

Authorities counted more than 50 separate eruptions early on Wednesday, but said villages outside the evacuated area were not in immediate danger.

Mount Sinabung is among about 130 active volcanoes in Indonesia, which is prone to seismic upheaval due to its location on the Pacific "Ring of Fire", an arc of volcanoes and fault lines encircling the Pacific Basin.

The 2,600-metre Mount Sinabung has sporadically erupted since 2010, after being dormant for 400 years. An eruption last year killed at least 17 people.

More than 2,700 people were evacuated from villages around the crater into several temporary shelters.