Cambodia's Prime Minister Hun Manet on Wednesday thanked Thailand for stopping "interference" in his country's politics, days after three Cambodian activists were held by Thai police.

Rights groups have long accused Cambodia's government of persecuting political opponents, and criticised Thailand for arresting dissidents on its soil.

Hun Manet held talks with his Thai counterpart Srettha Thavisin in Bangkok on Wednesday and thanked him for "not allowing Thai territory to be used for any activities for interference in Cambodian internal politics".

"The Cambodian government also gives its commitment for not allowing Cambodia's soil as land for conducting activities to interfere with Thailand or causing issues in Thailand," he added, standing alongside Srettha at a joint news conference.

Hun Manet said the arrangement promoted "stability, and trust and confidence" between the two neighbours.

Hun Manet's father Hun Sen handed over power last year after nearly four decades of ruling with an iron fist -- eliminating all opposition to his power, with rival parties banned, challengers forced to flee and freedom of expression stifled.

Human Rights Watch's deputy Asia director Phil Robertson condemned Thailand's cooperation with Phnom Penh, calling it a "black mark" against Srettha's record.

"They've declared an effectively open season on Cambodian political activists and refugees in Thailand," he told AFP.

Hun Manet was sending a loud signal to any remaining dissident voices inside or outside Cambodia, Robertson said.

"He's saying to them, you can run but you can't hide," he said.

Hun Sen's ruling party this week sued a prominent human rights defender for comments deemed as slander against it.

All three activists held last week appear to have UN refugee status, according to rights groups involved in their cases, but in the past this has not prevented Thailand from deporting people.

In 2021 the UN refugee agency, UNHCR, condemned Thailand for repatriating three Cambodian refugees in the space of two weeks -- despite the agency's attempts to intervene.

Emilie Pradichit, of rights group Manushya Foundation, condemned Thailand's cooperation and urged the international community to hold Srettha's government accountable.

"Thailand cannot continue to collaborate with Mekong governments in suppressing democracy and political activists inside its borders," she said.