More than 19,000 people have been displaced by a volcano in Indonesia that has been erupting for months and shot lava into the air nine times overnight, an official said Tuesday.

Mount Sinabung on the western island of Sumatra sent hot rocks and ash 7,000 metres (23,000 feet) in the air Monday night and Tuesday morning, National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho said.

"Mount Sinabung remains on the highest alert level and we have warned there should be no human activity within a five-kilometre (three-mile) radius of the crater," Nugroho said.

"On Monday night, 19,126 people had fled their homes, and we expect that number to rise," he said.

Police and soldiers were patrolling the danger zone to evacuate people who have chosen to stay in their homes, Nugroho said.

Mount Sinabung -- one of dozens of active volcanoes in Indonesia which straddles major tectonic fault lines, known as the Ring of Fire -- erupted in September for the first time since 2010 and has been rumbling ever since.

In August, five people were killed and hundreds evacuated when a volcano on a tiny island in East Nusa Tenggara province erupted.

The country's most active volcano, Mount Merapi in central Java, killed more than 350 people in a series of violent eruptions in 2010.