Indonesian authorities have closed an airport and residents left homes near an erupting volcano due to the dangers of spreading ash, falling rocks, hot volcanic clouds and the possibility of a tsunami.

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

The crater emitted white-grey smoke continuously during the day on Thursday, reaching more than 500 metres above the peak.

People have been ordered to stay at least six kilometres from the 725-metre mountain.

More than 11,000 people live in the affected area and were told to leave, and at least 800 have done so.

An international airport in Manado city was temporarily closed as volcanic ash was spewed into the air. On Friday, the closure was extended until 6pm local time (8pm AEST) as the eruptions continued.

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

Eruptions on Wednesday night spewed volcanic ash about 21 kilometres into the atmosphere, according to the Australian Bureau of Meteorology’s Volcanic Ash Advisory Centre.

Indonesia’s volcanology centre noted the risks from the volcanic eruption included 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 about three-quarters of 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 there is a collapse. Its residents were among those told to leave.

“People who live in the Tagulandang Island area and are within a six-kilometre radius must be immediately evacuated to a safe place outside the six-kilometre radius,” National Disaster Mitigation Agency spokesman Abdul Muhari said on Thursday.

“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 would 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.