Three rescue helicopters reached the crash site on the slopes of Mount Namal near the town of Gabaldon, 100km north of here and retrieved the bodies within hours, they added.

Among those killed were four scientists from the Philippine Institute of Volcanology and Seismology (Phivolcs) as well as their former boss, Raymundo Punongb-ayan, the country's most famous seismologist, according to Phivolcs chief Renato Solidum.

Punongbayan became a governor of the Filipino chapter of the Red Cross after retirement.

The helicopter's four-man crew were also killed, Solidum said.

The cause of the crash was not known, said the air force, who added that it happened under clear skies.

Senator Richard Gordon said the flight was organised by the Philippine National Red Cross to conduct an aerial inspection of a planned resettlement site for victims of last November's deadly landslides around the town of Dingalan, near Gabaldon.

"I received a message from the Office of Civil Defence that unfortunately, all the passengers died," said Phivolcs chief Renato Solidum.

"The aircraft exploded but we don't know if it was upon impact or while in mid-air," said Senior Inspector Pablo Cruz, police chief of Gabaldon, who was among the first at the crash scene.