Jakarta -Climbers and residents were warned to stay away from Mount Soputan volcano on Indonesia's Sulawesi island after it spewed smoke and flames from its crater, officials said Tuesday.

Volcanologists upgraded the alert status to the third of four levels after the 1,783-metre Soputan erupted Monday morning, sending smoke up to 1,000 metres into the sky and blanketing ash nearby.

There was no evacuation order but people were urged to stay outside a radius of 6 kilometres from the crater to avoid possible danger, said Sandy Manengkey, a volcanologist at a nearby monitoring post.

People were also barred from climbing the volcano or staying at the camping area on the north-east slope, about 4 kilometres from the crater, Sandy said, adding that the nearest village was 8 kilometres away.

A new lava dome has been forming at the peak of Mount Soputan volcano, about 2,160-kilometres north-east of Jakarta, since a big eruption in 1991, the state volcanology centre said on its website.

Experts warned that its collapse will send lava as well as deadly heat clouds speeding down its slopes.

The Indonesian archipelago, straddling the seismically active Ring of Fire, has the world's highest density of volcanos. Of its 500 volcanos, 128 are active and 65 are listed as dangerous.