A strong earthquake of magnitude 6.3 struck off the Philippine's Mindanao island on Saturday, sending hotel guests and construction workers running from buildings in Davao, but there were no immediate reports of damage.

The US Geological Survey said the quake, initially reported as a magnitude 6.5, struck at 6:53 a.m. (22:53 GMT on Friday) and was centered 114 km east of Davao on Mindanao.

It was a deep tremor, 69 km below the seabed, and was not expected to cause a tsunami, the Pacific Tsunami Warning Center said.

A magnitude 6.3 quake is considered strong and is capable of causing severe damage, but Renato Solidum, head of the Philippines' seismic agency, said the effects of this one would be mitigated by its location and depth.

"The quake is offshore and relatively deep and shaking is reported to be maximum of Intensity V, hence we expect no significant damage," Solidum told Reuters.

Dave Cabrera, a security guard at Hotel Uno in downtown Davao, said the quake was felt strongly in the city.

"Many guests left their rooms and rushed downstairs," he said. "That is the strongest I have felt for this year."

Around 1,000 construction workers building the 23-storey Avida Towers nearby also evacuated, said Junrey Quiroa, a security guard on the site.

"That earthquake was strong. We were worried things might fall off so everyone got out of the site."

The Philippines is on the geologically active Pacific Ring of Fire and experiences frequent earthquakes.-Reuters