The tremor late on Tuesday also injured many others and sent residents scrambling out of homes into darkness as the intense shaking cut off power, officials said.
The epicentre of the earthquake, which was set off by a local fault, was about 17km northeast of Bogo, a coastal city of about 90,000 people in Cebu province where at least 14 residents died, disaster-mitigation officer Rex Ygot told The Associated Press.
The death toll in Bogo was expected to rise. Workers were trying to transport a backhoe to hasten search and rescue efforts in a cluster of shanties in a mountain village hit by a landslide and boulders, he said.
"It's hard to move in the area because there are hazards," Glenn Ursal, another disaster-mitigation officer told AP, adding some survivors were brought to a hospital.
Six people, including three coast guard personnel, a firefighter and a child, were killed separately in San Remigio town, south of Bogo, the town's vice mayor, Alfie Reynes, told the DZMM radio network without elaborating how the victims died.
Reynes appealed for food and water, saying San Remigio's water system was damaged by the earthquake.
In Bogo, the quake damaged concrete walls of houses, a fire station and concrete and asphalt roads, firefighter Rey Ca単ete said.
"We were in our barracks to retire for the day when the ground started to shake and we rushed out but stumbled to the ground because of the intense shaking," Ca単ete told AP, adding that he and three other firemen sustained cuts and bruises.
A concrete wall in their fire station collapsed, Ca単ete said. He and fellow firefighters provided first-aid to at least three residents, who were injured by falling debris and collapsed walls.
Hundreds of terrified residents gathered in the darkness in a grassy field near the fire station and refused to return home hours after the earthquake struck in Bogo.
Several business establishments visibly sustained damages and the asphalt and concrete roads where they passed had deep cracks, Ca単ete said, adding that an old Roman Catholic church in Daanbantayan town near Bogo was also damaged.
Cebu Gov. Pamela Baricuatro said the extent of the damage and injuries in Bogo and outlying towns in the northern section of the province would not be known until daytime. "It could be worse than we think," he said in a video message posted on Facebook.
The Philippine Institute of Volcanology and Seismology briefly issued a tsunami warning and advised people to stay away from the coastlines in Cebu and in the nearby provinces of Leyte and Biliran due to possible waves of up to one metre.
Teresito Bacolcol, director of the institute, said the tsunami warning was later lifted with no unusual waves being monitored.
Cebu and other provinces were still recovering from a storm that battered the central region on Friday, leaving at least 27 people dead mostly due to drownings and falling trees, knocking out power in entire cities and towns and forcing the evacuation of tens of thousands of people.
The Philippines, one of the world's most disaster-prone countries, is often hit by earthquakes and volcanic eruptions due to its location on the Pacific "Ring of Fire", an arc of seismic faults around the ocean. The archipelago is also lashed by about 20 typhoons and storms each year.
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