A volcano in western Indonesia erupted on Sunday, belching a column of ash around three kilometres into the sky and forcing the evacuation of dozens of hikers, officials said.

Mount Marapi on the island of Sumatra, with a peak of 2,891 metres, is on the third-highest alert level of Indonesia's four-step system and authorities imposed an exclusion zone around its crater.

There were no immediate reports of damage or casualties after the eruption at 2.54 pm local time, but 42 hikers were unaccounted for.

A total of 70 hikers had checked in at two entry points to Marapi since Saturday via an online booking system, according to the West Sumatra natural resources conservation agency.

"The... agency team are conducting evacuations. Twenty-eight people have been evacuated and efforts are still being made (to evacuate) the rest. Hopefully there are no fatalities and everyone is safe," Dian Indriati, the interim head of the agency, said in a video statement.

Some hikers may not notify authorities about their presence when entering or leaving such areas, so the true number of unaccounted hikers remains unclear.

Hendra Gunawan, head of Indonesia's Centre of Volcanology and Geological Hazard Mitigation, in a statement said ash was observed 3,000 metres above Marapi's peak.

"The column of ash was observed in grey colour with thick intensity leaning toward the east," Gunawan said. "The eruption is still ongoing."

Communities around the Marapi volcano and tourists are not allowed to hike the Marapi volcano within a three kilometre radius from its crater.

Mount Marapi, which means 'Mountain of Fire', is the most active volcano on Sumatra. (AFP)