A Cambodian journalist who exposed rampant illegal logging has been found murdered in the boot of his car, police said Wednesday, in a country where environmental activists often face violent retribution.

Hang Serei Oudom, a reporter at local-language Vorakchun Khmer Daily, was discovered on Tuesday, said senior police officer Song Bunthanorm. The vehicle was abandoned in a cashew nut plantation in northern Ratanakiri province.

"It is not a robbery case. It is a murder," he said, adding that the victim had suffered several blows to the head, probably with an axe.

The 44-year-old had been missing since leaving his home on Sunday evening.

"He wrote stories about forest crimes involving business people and powerful officials in the province," said Vorakchun Khmer Daily editor-in-chief Rin Ratanak, adding most of his stories were about "illegal logging of luxury wood".

Rampant illegal logging contributed to a sharp drop in Cambodia's forest cover from 73 percent in 1990 to 57 percent in 2010, according to the United Nations.

Local activists said fellow journalists had recently started to fear for Oudom's safety, as a result of a string of stories he wrote about deforestation and timber smuggling in the province.

In his latest story, posted on the newspaper's website on September 6, Oudom accused the son of a military police commander of smuggling logs in military-plated vehicles and extorting money from people who were legally transporting wood.

"Before he was murdered, other journalists had warned him not to write critically about the forest crimes," said Pen Bonnar, provincial coordinator for rights group The Cambodian Human Rights and Development Association.

He said Ratanakiri was "a dangerous area" for reporters and activists working to combat forest crime, adding that illegal logging was linked to powerful and rich individuals in the region.

In its haste to develop the impoverished nation, the Cambodian government has been criticised for allowing well-connected firms to clear hundreds of thousands of hectares (acres) of forest land -- including in protected zones -- for everything from rubber and sugar cane plantations to hydropower dams.

Rights groups and environmental watchdogs have linked many of these concessions to illegal logging, and say armed government forces are routinely used to act as security guards for offending companies.

"How many more campaigners have to die before Cambodia's donors and the UN insist that their ongoing support requires the government to act to thoroughly investigate these cases and end impunity," said Phil Robertson deputy director of Human Rights Watch Asia.

The murder also threatens freedom of expression in the kingdom, according to Ramana Sorn of the Cambodian Center for Human Rights, saying it sends "a strong, intimidating message" to the Cambodian media "to practise self-censorship".

In late April, prominent environmentalist Chhut Vuthy was shot dead by a military policeman after he refused to hand over pictures showing logging in southwestern Koh Kong province.

Vuthy championed grassroots activism, including forest patrols by communities who depend on the woodlands for their survival.

Campaigners said the patrols burnt loggers' caches of luxury timber worth tens of thousands of dollars.

Following the outcry over Vuthy's death, Prime Minister Hun Sen ordered a freeze on new land grants, a move cautiously welcomed by environmental groups, who nevertheless argue it will not save the forests already under threat.