A powerful 6.1-magnitude earthquake struck off the coast of Mindanao in the southern Philippines today, the US Geological Survey (USGS) said.

The quake hit 160 km south-east of General Santos City at a depth of 49.6 km, according to the USGS.

No tsunami alert was immediately issued and there were no reports of damage or casualties.

The Philippine Institute of Volcanology and Seismology put the magnitude at 6.3 and said it was felt in the nearby city of Davao.

"We are expecting some aftershocks but the quake is too far out at sea to cause any significant damage," a spokeswoman for the agency told AFP.

The archipelago nation sits on the Pacific Ring of Fire where continental plates meet, causing frequent seismic and volcanic activity. - AFP