A strong 6.0-magnitude earthquake struck off Indonesia's Sumatra island early this morning , the US Geological Survey said.

The quake hit at 07:01am (0531 IST) and was centred 224 kilometres south-southwest of the town of Sibolga at a depth of 35 kilometres under the sea, the USGS said.

Local meteorologists said the quake measured 5.6 on the Richter scale and struck at a depth of 51 kilometres.