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The Bell UH-1H aircraft pitched into Mount Sinumaan on Jolo island when its crew attempted to land at a military outpost there, air force spokesman Lieutenant-Colonel Miguel Ernesto Okol said.
"The UH-1H attempted a precautionary landing... due to mechanical problems," he said in ad statement. He did not say what had caused the mechanical problem.
"There were three reported fatalities (but) the co-pilot survived," he said.
All the casualties were aboard the helicopter.