The crater of Mount Ruang flamed with lava against a backdrop of lightning bolts overnight after erupting four times on Wednesday, forcing authorities to raise its alert level to the highest of a four-tiered system.

The volcano in Indonesia's outermost region was still billowing a column of smoke on Thursday morning, prompting authorities to shut the nearest international airport in Manado city on Sulawesi island for 24 hours.

Authorities said they were rushing to evacuate 11,000 residents from the nearby area that included the remote island of Tagulandang, home to around 20,000 people.

Some residents were already trying to flee in a panic, according to officials.

"Last night people evacuated on their own but without direction due to the volcano's eruption and materials in the form of small rocks that fell, so the people scattered to find evacuation routes," Jandry Paendong, an official from the local search and rescue agency, said in a statement Thursday.

He said 20 staff were helping evacuate residents along the coastline near the volcano on rubber boats.

He called for more boats and equipment so his team could "carry out evacuation for people in the coast or near the coast" facing the volcano.

Tourists and residents were warned to remain outside a six-kilometre exclusion zone.

More than 800 people were initially taken to safety from Ruang to nearby Tagulandang island after the first eruption on Tuesday evening before four more eruptions on Wednesday.

- Tsunami warning -

Authorities also warned of a possible tsunami as a result of the eruptions.

"The communities in Tagulandang island, particularly those residing near the beach, (need) to be on alert for the potential ejection of incandescent rocks, hot clouds discharges and tsunami caused by the collapse of the volcano's body into the sea," Hendra Gunawan, head of Indonesia's volcanology agency, said in a statement Wednesday.

The authorities' fears were compounded by previous experience.

In 2018, the crater of Mount Anak Krakatoa between Java and Sumatra islands partly collapsed when a major eruption sent huge chunks of the volcano sliding into the ocean, triggering a tsunami that killed more than 400 people and injured thousands.

Indonesia, a vast archipelago nation, experiences frequent seismic and volcanic activity due to its position on the Pacific "Ring of Fire", an arc where tectonic plates collide that stretches from Japan through Southeast Asia and across the Pacific basin.

The impact of Mount Ruang's eruption led to the closure of Sam Ratulangi International Airport in Manado city, located more than 100 kilometres (62 miles) from the volcano, for 24 hours until Thursday evening.

The airport runways were shut "due to the spread of volcanic ash which could endanger flight safety," Ambar Suryoko, head of the Manado region airport authority office, said in a statement.

The airport hosts airlines that fly to Singapore and cities in South Korea and China.

"All flights... impacted because the airport is affected by the eruption of Mount Ruang, volcanic ash," Dimas, 29-year-old airport officer in Manado told AFP by phone.