North Sumatra police have arrested an oil palm plantation owner believed to be the mastermind behind the murder of environmental activists Maraden Sianipar and Maratua Siregar.

The two men were found murdered in a ditch near an oil palm plantation run by PT Sei Alih Berombang in Wonosari village on Oct 30.

AFP reports that businessperson Wibharry Padmoasmolo has been arrested for allegedly masterminding the plot by paying almost US$3,000 to four men to commit the killings.

According to the Jakarta Post, North Sumatra Police chief Agus Andrianto said it was believed the murders were triggered by a land dispute between the company and local farmers.

Agus said Maraden and Maratua came to the plantation and were met by four armed men.

They were killed after a brief quarrel, he added.

He said that suspect Wibharry is now denying owning the business, claiming his father-in-law was one of the owners instead.

Before working as freelance journalists, Maraden and Maratua worked for local weekly Pilar Indonesia Merdeka (Pindo Merdeka).

Unesco Director-General Audrey Azoulay condemned the killings and called on authorities to bring their perpetrators to trial.

"Justice must be mobilised to stop violent attacks on journalists whose work is essential to both democracy and rule of law," she said on Friday.