Communist rebels waging a long-running insurgency in the Philippines freed Monday a group of forestry surveyors they had seized on suspicion of being government spies.

The New People's Army (NPA) rebels said they had abducted the five at a remote mountainous area on the southern island of Mindanao after they were spotted flying drones over the area.

The group had been on its way to study a government forestry programme in the area when they were taken Friday.

Daniel Ibarra, a spokesman for the local NPA unit, said Monday the five had been spotted flying drones over guerrilla-controlled territories.

He claimed they were engaged in "counter-revolutionary intelligence" gathering.

The NPA is the armed wing of the Communist Party of the Philippines, which has been waging a Maoist rebellion that has killed tens of thousands since 1969.

The rebels confiscated three remote-controlled drones from the group, laptops and a global positioning satellite device.

The military had earlier reported six were taken, but subsequently said there were only five in the group.

A local city mayor, Rodrigo Duterte, negotiated their release with the NPA.

"Thy were freed earlier after we established contact," Duterte's aide, Christopher Go, said.

"They were mistaken as government spies, but we explained that they were really commissioned by the environment department."