A series of eruptions at an Indonesian volcano have thrown rocks, lava and ash hundreds of metres into the air and forced thousands of people to flee, officials said Friday.

Mount Lokon on Sulawesi Island burst into life overnight Thursday and continued to spew ash and searing gas into the air well into the morning, prompting more than 4,500 residents to head to safety.

"There was a big eruption around 10:31pm local time (1531 GMT Thursday), which saw ash, sand and rocks thrown 1,500 metres (4,800 feet) into the air," government volcanologist Kristianto told AFP.

Grass and shrubs within 500 metres (yards) of the crater caught fire but by mid-morning on Friday the flames were out and the volcano had calmed down, he said.

Hundreds of people fled their homes in the middle of the night when they were woken by a thunderous explosion and flashes of lightning.

"I was sitting in a security post with other villagers when I suddenly heard an extremely loud explosion, it was like big thunder. I also saw lightning in the sky," one resident of Kinilow village, about three kilometres (1.8 miles) from the volcano, told AFP.

"People who were sleeping were woken up and rushed to safety."

Villagers jumped on motorcycles and piled into trucks and buses as police with loudspeakers urged them to run for their lives.

Around 28,000 people live within the evacuation zone.

"There is no mass panic because the community has already been warned of the situation and we are continuing to evacuate people," said Kristianto, who goes by one name.

Search and rescue official Yohanes Sondang said evacuees were staying in six schools and would be barred from returning to their homes as long as the alert level remain high.

"We've deployed about 200 personnel including policemen and soldiers to stop people who try to return to their houses," he said.

Patrols have also been sent into the evacuation zone to protect property and ward off looters, he added.

Aviation officials said pilots had been warned to avoid routes over the volcano but all airports including the international strip at Manado were operating normally.

The volcano's status was raised to its highest red alert level on the weekend and a 3.5-kilometre evacuation zone was established.

North Sulawesi disaster management head Hoykee Makarawung said more than 4,400 people had been evacuated so far.

"Thank God, the condition has eased off since last night. The fire around the area has also vanished," he said.

The 1,580-metre Mount Lokon is one of the most active volcanoes in Indonesia. It erupted in 1991, killing a Swiss tourist.

The Indonesian archipelago has dozens of active volcanoes and straddles major tectonic fault lines known as the "Ring of Fire" between the Pacific and Indian oceans.

The country's most active volcano, Mount Merapi in central Java, killed more than 350 people in a series of violent eruptions last year.