An international airport in Manado city, which is less than 100km from the erupting Mount Ruang, is still temporarily closed as volcanic ash was spewed into the air.

Satellite imagery from the Meteorology, Climatology and Geophysics Agency showed that the ash has spread to the west, northwest, northeast and southeast, covering Manado and North Minahasa, according to a statement from Indonesia's transportation ministry.

"We are still monitoring developments in the eruption of Mount Ruang and co-ordinating with relevant stakeholders ... to anticipate the necessary actions to ensure flight safety, security and comfort," said Ambar Suryoko, head of the regional airport authority.

More than 11,000 people were told to leave their homes and at least 1000 have done so.

A joint team from the local authorities is still combing the villages surrounding the volcano and moving the residents to safer areas by boat.

Officials worry that part of the volcano could collapse into the sea and cause a tsunami, as happened in an eruption there in 1871 eruption.

Houses, roads and other buildings in the affected areas were covered by grey volcanic ash.

Many house roofs were also broken by the materials spewed from the eruption.

Mount Ruang saw at least five large eruptions on Wednesday, causing the Centre for Volcanology and Geological Disaster Mitigation to issue its highest level of alert.

People were ordered to stay at least 6km from the 725-metre mountain.

The observation from the agency on Friday said that white smoke was rising from the main crater with medium to thick intensity.

Tagulandang Island, east of the volcano, could be at risk if a collapse occurred.

Its residents were among those being told to leave.

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 series of seismic fault lines around the Pacific Ocean.

Australian Associated Press