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A court in Cambodia has charged three environmental activists with plotting against the government and insulting the king.

The members of the group Mother Nature were arrested after they documented waste run-off into Phnom Penh's Tonle Sap river.

Cambodia's law against insulting the king is relatively new and it is not clear how the activists have done so.

The three face between five and 10 years in prison.

The prosecution told the Reuters news agency that "the evidence collected by the police was an insult to the king" although they did not elaborate exactly how the law had been violated.

Critics have warned that the law – introduced in 2018 – is being used as a tool to silence dissent.

The activists are Sun Ratha, 26, Ly Chandaravuth, 22, and Yim Leanghy, 32, all members of activist group Mother Nature.

They were detained on June 16 while documenting waste discharge into the Tonle Sap river near the royal palace, according to the Cambodian Center for Human Rights.

"The Cambodian government has relentlessly targeted Mother Nature Cambodia," rights group Licadho director Naly Pilorge told Reuters.

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

Mother Nature founder Alejandro Gonzalez-Davidson, who was deported in 2015 after he criticised plans for a controversial dam, was charged in absentia.

Last month, three other campaigners linked to the group were sentenced to between 18 and 20 months in jail for organising a march against a lake in the capital being filled with sand.

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The embassies of Sweden and the United States have both criticised the charges.

Sweden is concerned about recent legal actions against young environmental activists in Cambodia. Civil engagement, access to information, transparency and dialogue key components for sustainable development. Engaged youth is part of the future and should be encouraged.

— Sweden in Cambodia (@SwedeninKH) June 21, 2021
The BBC is not responsible for the content of external sites.View original tweet on Twitter

"Very troubled to hear of the arrests of more environmental youth activists," US ambassador Patrick Murpy said. "Documenting pollution is a public service, not terrorism. We urge authorities to be responsive to its citizens, not to silence them.

"The Cambodian government has stepped up its campaign to silence activists peacefully advocating to protect the environment," Phil Robertson, deputy Asia director at Human Rights Watch (HRW) said.

"Foreign governments, the United Nations country team, and international donors should call on the Cambodian authorities to drop their absurd charges against the environmental activists and publicly condemn any further clampdown on peaceful activism."

"Cambodia's highly politicised courts mean that the environmental activists charged have no chance of getting a fair trial," he added.

image copyrightGetty Imagesimage captionIt's not clear how the activists "insulted" Cambodia's kind Norodom Sihamoni

Government spokesman Phay Siphan dismissed criticism of the charges, saying the government was simply applying the law.

Defendants should "find a good lawyer to challenge this issue in the courtroom instead of fabricating the news," he said.

Cambodia is a constitutional monarchy, with almost all power in the hands of Prime Minister Hun Sen who has run the country for decades.

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The authoritarian government is known for cracking down harshly on criticism and dissent.