A strong undersea tremor hit southern Philippines Thursday but no tsunami alert was issued, the US Geological Survey (USGS) said.

The 6.4-magnitude quake, which occurred around 3:31 p.m. local time, had a depth of 618.8 km. The epicentre was estimated at 125 km southwest of Cotabato city, Xinhua reported.

No casualties were reported.

The Philippines sits on the Pacific 'Ring of Fire' where continental plates collide causing frequent seismic and volcanic activity. The worst earthquake in the country happened Aug 16, 1976 when a tsunami killed between 5,000 and 8,000 people in the Moro Gulf region in southern Philippines.