Jakarta: The murder of two Indonesian activists, allegedly ordered by a palm oil businessman, highlights the threats faced by environmentalists, campaigners say.

Martua Parasian Siregar, 55, and Maraden Sianipar, 42, were found dead 10 days ago with multiple stab wounds near a palm plantation in the island of North Sumatra.

Indonesian police have arrested the head of the Amelia palm oil company, who they identified only as "H" or "Harry", on suspicion of having paid several men about $3000 to kill the two activists.

"This strengthens our suspicion that human rights violations in palm oil companies are rampant," said Dana Prima Tarigan, who heads green group the Indonesian Forum for the Environment.