A strong 6.1-magnitude quake struck off Indonesia's Sulawesi island on Friday, seismologists said, but there were no immediate reports of damage and no tsunami warning was issued.

The quake struck at 1833 GMT on Friday at sea 216 kilometres northeast of Baubau, in the southeast Sulawesi region at a shallow depth of 10 kilometres, the US Geological Survey said.

Indonesia sits on the Pacific "Ring of Fire," where the meeting of continental plates causes high volcanic and seismic activity.

A 7.6-magnitude earthquake jolted the country last month, killing 1,115 people and leaving an estimated half a million homeless.

An expert predicted this week a colossal earthquake may hit Indonesia within 30 years, triggering a tsunami and making last month's deadly quake look tiny by comparison.

Kerry Sieh, director of the Earth Observatory of Singapore, said the next big quake would last more than six times as long as the 7.6 magnitude quake which struck western Sumatra on September 30, levelling the city of Padang.

A massive tsunami hit Indonesia and other countries in the Indian Ocean rim in 2004, killing about 220,000 people, most of them in Aceh province in northern Sumatra.