By Karen Lema

MANILA (Reuters) - A powerful earthquake struck near the Philippines city of Davao on Sunday, the U.S. Geological Survey said, the latest tremor to strike the southern part of the country in recent months, causing damage to buildings but no tsunami.

The magnitude 6.8 quake was centred 61 km (38 miles) southwest of Davao on the island of Mindanao, at a depth of 28.2 km, the USGS said, revising down the magnitude from an earlier 6.9.

The Pacific Tsunami Warning Center said there was no tsunami threat from the quake, based on available data.

There were no immediate reports of injuries, but the Philippine Institute of Volcanology and Seismology (Phivolcs) said damage and aftershocks could be expected.

Two strong aftershocks of magnitude 5 and 5.7 centred north of General Santos City was recorded after the main tremor.

Radio DZMM reported power was out in General Santos City, where patients at a local hospital were being evacuated.

Video footage of the latest quake posted by users on Twitter showed overhead electrical wires swaying and erupting in a shower of sparks. Another video showed water sloshing out of a residential swimming pool.

One picture said to be from Padada, close to the epicentre in the province of Davao del Sur, showed a building collapsed onto a car. Reuters could not immediately verify the images.

Mindanao was shaken by four powerful quakes in October and November, which together killed at least 20 people.

The Philippines sits on the geologically active Pacific "Ring of Fire" and experiences frequent tremors.

(Aditional reporting and writing by Lincoln Feast in Sydney; Editing by Muralikumar Anantharaman)