PHNOM PENH (REUTERS) - A Cambodian court has charged four environmental activists with insulting the country's King and conspiracy to plotting, a prosecutor said on Monday (June 21), following the arrest of three of them last week as they documented waste discharge into a city river.

Deputy Prosecutor Plang Sophal of Phnom Penh Municipal Court confirmed the charges in a text message but did not specify how the activists, from the group Mother Nature, had broken the laws.

"The evidence collected by the police was an insult to the King," he said, without elaborating.

Though filed often in Thailand, charges of lese majeste, or insulting the monarchy, are rare in Cambodia.

The four face up to 10 years in prison if found guilty of plotting and up to five years in jail for royal insults.

Mother Nature's Spanish founder Alejandro Gonzalez-Davidson is overseas and was charged in absentia. He said the charges were "completely fabricated", and reflected government paranoia about its own citizens.

"As for the charges against me, I see them as further recognition that the regime sees my peaceful activism as a threat," he said in a text.

Mother Nature's Sun Ratha, 26, Ly Chandaravuth, 22, and Yim Leanghy, 32, were detained last Wednesday while documenting run-off into the Tonle Sap river in Phnom Penh, said human rights monitor Licadho.

"The Cambodian government has relentlessly targeted Mother Nature Cambodia," said Licadho's director Naly Pilorge.

"This marks an escalation with the outrageous charges of 'plotting'."

The government's spokesman Phay Siphan said it was applying the law and that the defendants should "find a good lawyer to challenge this issue in the courtroom instead of fabricating the news".

The United States last week scrapped a wildlife conservation programme with Cambodia for what it said was a failure to tackle logging and intimidation of environmentalists.