Cambodia's prime minister has ordered a million hectares of forest be included in protected zones as the country faces one of the world's fastest deforestation rates.

The move, which covers five new areas of forest, will bump Cambodia's conservation zones up by a fifth, bringing more than a quarter of the country's land under protection.

"The Ministry of Environment must... list the five forests as protected areas," said the order signed by Prime Minister Hun Sen, which was seen by AFP on Saturday.

The new conservation areas will include parts of Prey Lang -- a forest where activists have long been risking their lives to expose the illegal logging that has eviscerated Cambodia's forest cover.

The lucrative trade, lubricated by violence and bribery of forestry officials and border guards, has contributed to the clearance of around one third of the country's forested land in the past 30 years.

Hun Sen has been in power throughout that time, but conservationists say he has made little headway in reducing illegal logging despite trumpeting several crackdowns.

His government has also been criticised for allowing firms to clear hundreds of thousands of hectares of forest land -- including in protected zones -- for everything from rubber and sugar cane plantations to hydropower dams.

Other forests named in the new order are Prey Preah Roka, Prey Siem Pang Khang Lech, Prey Chrak Robeang Khang Tbong and Prey Veun Sai -- all of which were previously administered by the Ministry of Agriculture, Forestry, and Fisheries.

The NGO Conservation International welcomed the new protections, calling the decree a "bold move".

"These sites represent the most important forests in Cambodia for biodiversity conservation and support of human wellbeing, and if managed correctly could lead to a paradigm shift in Cambodia's development pathway," said Tracy Farrell, the regional director of CI's Greater Mekong program.

According to the organisation, Cambodia's forests provide refuge to over 800 globally threatened species, more than half of which depend on forests to survive.

Many local communities also depend on the woodlands for their livelihoods.

Last week Cambodian authorities banned a documentary about a high-profile land activist, Chhut Vuthy, who was shot dead by a military policeman while investigating deforestation in a remote area in 2012.

In November 2015 a forest ranger and a policeman who were investigating illegal logging were killed.

At least 10 people, including a soldier, were arrested over the murders.