Jakarta: A 5.9-magnitude earthquake struck the Indonesian island of Sulawesi on Monday, US seismologists said, but there were no immediate reports of casualties or damage, and no tsunami alert.

The quake hit at 9:23 am (local time) at a depth of 33 kilometres, around 57 kilometres southwest of the main city of Palu, the US Geological Survey said.

"There was some shaking but no major panic in Palu. There's no report of casualty or injury so far," said Suharjono, technical chief at Indonesia's Meteorology and Geophysics Agency.

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