Mount Lokon, on Sulawesi, started erupting at around 1035 local time (0335 GMT) on July 17. It spewed hot ash and smoke 3,500-metres (11,500ft) into the air, according to reports.

There were no immediate reports of casualties.

Before the eruption, residents had been returning to their villages along the mountain's slopes, despite warnings to stay away from the crater. They were forced back into cars and motorcycles and many raced down the slopes to makeshift camps near the base.

A local plantation worker, Henry Lalawi, described the eruption as "huge".

The nearby international airport at Manado is operating normally but airlines travelling within 10km (six miles) of Mount Lokon's peak have been told to reroute their flights.

There are 28,000 people living within a 3.5-km (two mile) evacuation zone which was established on north Sulawesi last weekend.

Government volcanologists say the eruption is the biggest since late June.