The UN rights chief said Friday he was "deeply concerned" by a Cambodian court rejecting the appeal of detained opposition leader Kem Sokha against his nearly three-decade sentence for treason.

The 72-year-old politician was convicted of trying to topple the government of long-ruling former prime minister Hun Sen, who is the father of the current leader and remains an influential figure in national politics.

Kem Sokha was sentenced in 2023 to 27 years in prison for treason but was ordered to serve the time under house arrest in the capital Phnom Penh.

Volker Turk, the United Nations' high commissioner for human rights, said he we "deeply concerned by the upholding of the conviction" by the Phnom Penh appeals court on Thursday.

The co-founder of the dissolved opposition Cambodia National Rescue Party, Kem Sokha has repeatedly denied the charge since his 2017 arrest in connection with a speech he gave in Australia four years earlier.

Rights groups have long accused Cambodian authorities of using legal cases as a tactic to silence opposition voices and legitimate political dissent.

'Chilling effects'

Turk also voiced concerns about the convictions of 33 other opposition activists, human rights defenders and social media users, in a separate case on Wednesday.

The Phnom Penh first instance court imposed sentences ranging from 18 months suspended to two years in prison, Turk's OHCHR office said, on charges of "incitement to cause social chaos" over public comments they made in 2024 about the Cambodia-Laos-Vietnam Development Triangle Area project.

"These convictions and sentences are clearly inconsistent with international human rights law and risk further deepening the already-considerable chilling effects of broad, vaguely-worded criminal laws and their arbitrary enforcement on civil society, journalists and the broader population in Cambodia," OHCHR spokesman Jeremy Laurence said.

"Kem Sokha and the other 33 individuals were all exercising their rights to freedom of expression. Their trials also raise concerns about violations of due process and fair trial rights," he told a press conference in Geneva.

Turk urged Cambodia to ensure that legitimate criticism and expression are protected rather than criminalised, to safeguard civic space, to guarantee the independence of the judiciary and uphold fair trial guarantees.

"The authorities should promptly quash these latest convictions and sentences, and unconditionally release Kem Sokha and all others arbitrarily detained for exercising their rights," said Laurence.

Laurence said OHCHR had consistently raised concerns with the Cambodian authorities "on general repression, and particularly on freedom of speech, expression, association and assembly".