The victims had camped on the flanks of the Mount Mayon, planning to reach the summit crater to view its pool of molten lava.

Seven other climbers who sustained severe injuries from the falling debris had to be airlifted by helicopter to area hospitals.

The blast was not due to an eruption, according to Philippine vulcanologists.

Renato Solidum Jr., Director of the Philippine Institute of Volcanology and Seismology, told the Philippine Star that the explosion was caused when rainwater came in contact with super-heated material inside the mountain.

The blast sent a plume of ash and steam soaring about 1,600 feet into the air.

Mayon is located about 200 miles southeast of Manila and has erupted 48 times in recorded history.

In 1814, more than 1,200 people were killed when lava flows buried the nearby town of Cagsawa.

Photo: Bert Recamunda