Three suspected timber traders have been arrested over the murder of a Cambodian journalist investigating illegal logging in the country's forested east, police said Monday.

Taing Try, 48, was shot dead early on Sunday, according to Oum Phy, deputy police chief of Kratie province.

He is the second journalist probing Cambodia's lucrative trade in illegal timber to be killed in two years.

A former soldier, a police officer and a Phnom Penh-based military police officer -- all suspected log traders -- were arrested several hours later.

Oum Phy said the 32-year-old ex-soldier was the chief suspect in the killing, although police are still investigating the motive.

"He (the slain reporter) may have damaged the interests of the three suspects," Oum Phy added.

Sok Sovann, president of the Khmer Journalists for Democracy Association, said Taing Try was killed while he and several other reporters were investigating illegal logging -- which is widespread in the impoverished nation.

"He went to the area where he was told illegal logs were being transported," he told AFP, adding that Taing Try contributed to several local newspapers.

Sok Sovann denied local media reports that the slain journalist was trying to extort money from the suspects.

He said Taing Try had been repeatedly accused of extorting money from timber traders in the past -- although there was no evidence to back up the allegations.

Environmental activists regularly face threats in Cambodia, where land grabbing has become a major source of tension and illegal logging is rampant.

In April 2012, prominent environmentalist Chhut Vuthy was shot dead in a remote forest by a military policeman after he refused to hand over pictures showing logging in the southwestern Koh Kong Province.

The officer who shot Vuthy was then himself accidentally killed with his own weapon when a private security guard tried to disarm him, according to a government probe.

Less than six months later, Hang Serei Oudom -- a reporter at local-language Vorakchun Khmer Daily -- who also exposed illegal logging was found dead in the boot of his car in northern Ratanakiri Province.

A Cambodian court acquitted a military policeman in 2013 over Oudom's brutal killing.

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 of forest land -- including in protected zones -- for everything from rubber and sugar cane plantations to hydropower dams.

Unchecked 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.