Mount Ruang on the northern side of Sulawesi Island had at least five large eruptions Wednesday, causing the Centre for Volcanology and Geological Disaster Mitigation to issue its highest-level alert, indicating an active eruption.

The crater emitted white-gray smoke continuously during the day Thursday, reaching more than 500 metres (1,600 feet) above the peak.

People have been ordered to stay at least 6 kilometres (3.7 miles) from the 725-metre (2,378 foot) mountain. More than 11,000 people live in the affected area and were told to leave. At least 800 have done so.

An international airport in Manado city was temporarily closed Thursday as volcanic ash was spewed into the air.

“We have to close flight operations at Sam Ratulangi Airport due to the spread of volcanic ash, which could endanger flight safety,” said Ambar Suryoko, head of the regional airport authority.

Eruptions Wednesday evening spewed volcanic ash approximately 70,000 feet into the atmosphere, according to the Australian Bureau of Meteorology's Volcanic Ash Advisory Centre. The bureau said in a statement Thursday it was tracking and forecasting the ash dispersion.

Indonesia's volcanology centre noted the risks from the volcanic eruption include the possibility that part of the volcano could collapse into the sea and cause a tsunami. In December 2018, Indonesia's Anak Krakatau volcano island erupted and collapsed, losing around 3/4 its volume and triggering a powerful tsunami that killed more than 400 people. An 1871 eruption at Mount Ruang also triggered a tsunami.

Tagulandang Island, east of the Ruang volcano, could be at risk if a collapse occurred. Its residents were among those being told to evacuate.

“People who live in the Tagulandang Island area and are within a 6-kilometre radius must be immediately evacuated to a safe place outside the 6-kilometre radius," Abdul Muhari, spokesperson of the National Disaster Mitigation Agency, said Thursday. “And especially those who live near the coast should be aware of the potential for incandescent rocks to erupt, hot clouds and tsunami waves that could be triggered by the collapse of a volcanic body into the sea.”

The agency said residents will be relocated to Manado, the nearest city, on Sulawesi island — a six-hour journey by boat.

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.