(CNN) -- Rescue workers are searching for more than 300 Filipino fishermen who have gone missing after a devastating typhoon that tore across the Philippines last week, killing more than 600 people, authorities said Monday.
Typhoon Bopha, the strongest tropical cyclone to hit the Philippines this year, set off flash floods and landslides that engulfed people sheltering in their rickety houses in remote, unprepared regions on the southern island of Mindanao.
The number of people still unaccounted for climbed sharply during the weekend as officials learned of the disappearance of boatloads of fishermen who set off before Typhoon Bopha slammed into the east coast of Mindanao on Tuesday.
Rescue efforts are under way to try to find the missing fishermen, many of whom left from the port city of General Santos on the south coast of Mindanao, said Maj. Reynaldo Balido, military assistant for operations at the Philippine Office of Civil Defense.
A total 780 people are missing in the Philippines as a whole, the National Disaster Risk Reduction and Management Council, the country's emergency management agency, said Monday.
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That includes the fishermen and more than 400 people in New Bataan, a town in southeastern Mindanao where flash floods triggered by Bopha's heavy rain buried large areas under mud, rocks and trees.
The overwhelming majority of the 647 people killed by the typhoon are from the regions of Davao Oriental and Compostela Valley, where New Bataan is situated.
Many people in those areas failed to anticipate Bopha, known locally as Pablo, the strongest storm to hit Mindanao in decades. The island experiences fewer tropical cyclones than the regions of the Philippines further north.
Survivors are now struggling to find enough water, food and shelter. Relief agencies are scrambling to provide help to the worst hit areas, but damage to infrastructure like roads and communications networks is complicating their task.
The scale of the challenge facing them is huge: nearly 1,500 people have been injured and more than 300,000 have been forced to seek shelter in evacuation centers or elsewhere.
Philippine President Benigno Aquino III, who visited New Bataan and other severely affected areas Friday, has declared a state of national calamity, which releases emergency funds and puts price controls on basic goods.