Islands in Indonesia’s North Sulawesi region are on alert for a tsunami as volcano eruptions send lava spewing and a mile-high ash cloud forms above Mount Ruang.

Authorities ordered thousands of people to evacuate and the volcano alert was raised to the country’s highest level on Wednesday after five large eruptions were recorded in one day, Indonesia’s Centre for Volcanology and Geological Disaster Mitigation said.

Indonesia, an archipelago of 270 million people, has 120 active volcanoes. It is prone to volcanic activity because it sits along the “Ring of Fire,” a horseshoe-shaped series of seismic fault lines around the Pacific Ocean.

The volcanic activity, which began late on Tuesday night and followed two earthquakes in the region, sparked fears of a repeat of the 1871 disaster when hundreds of people died in a tsunami in the days after Mount Ruang erupted.  

On Wednesday morning, 800 residents fled by boat from Ruang to nearby Tagulandang. But by Wednesday night, that island was no longer considered safe either. 

Indonesia’s National Disaster Mitigation Agency said 11,000 people would be relocated to Manado, the nearest city, on Sulawesi island, a journey of six hours by boat.

An exclusion zone was extended to prevent people coming within 4 miles of Ruang. 

“Beware of large waves due to the impact of the eruption of Mount Ruang for communities in the coastal areas of Tagulandang Island, Biaro, Siau Island and North Minahasa Beach,” a spokesman for the BMKG Earthquake and Tsunami Centre told Kompas newspaper.

‘‘(The potential cause of tsunami) is mainly the flank collapse phenomenon or the collapse of part or all of the mountain body.’’