Thavry Lanh — who represented the now dissolved Cambodian opposition party at district government level and was a vocal critic of Prime Minister Hun Sen's government — fled to Thailand more than four years ago.

Thai immigration officers arrested her on Friday in an eastern border town and she was sent to Cambodia the next day.

Gillian Triggs, a senior official with the UN High Commission for Refugees (UNHCR), was quoted by AFP as saying that the move contravened the principle of nonrefoulement, under which countries are banned from sending people back to places where their lives or freedom are in danger.

Thavry Lanh's deportation came 10 days after two other Cambodian UNHCR-registered refugees were sent across the border, the agency said in a statement issued on Monday.

The UNHCR tried to intervene in all three cases but the Thai government went ahead with the deportations.

"We are extremely alarmed by this trend of forcibly returning refugees to Cambodia, where they face a serious risk of persecution," Triggs said in the statement, adding that the agency was worried about the fate of other UNHCR-registered refugees in Thailand.

Thavry Lanh's husband Radong Phin said he had grave fears for his wife's safety and wellbeing.

She was in custody at Prey Sar prison in Phnom Penh, awaiting trial for "treason" but the court date was unknown, he said.

"She might be incarcerated for months or years. I'm also worried about her health because she has asthma and takes medicine," he told AFP, adding he had not been able to speak to her because her phone was confiscated.

AFP has contacted the Thai Foreign Ministry for comment.

Thailand and Cambodia made an agreement to exchange "fugitives" in 2018.

Earlier this month, Veourn Veasna and Voeung Samnang, who both have links to the disbanded Cambodia National Rescue Party, were returned from Thailand to face "politically motivated charges," Human Rights Watch (HRW) said.

Both men had posted material critical of the Hun Sen government on Facebook and had fled to Thailand last year.

"It's open season now on Cambodian refugees," HRW's Asia deputy director Phil Robertson told AFP.

Hun Sen is one of the world's longest-serving leaders and has been in power for 36 years.

Critics say he has wound back democratic freedoms and used the courts to stifle opposition and activists.

The Cambodian government is hosting an online summit of European and Southeast Asian leaders later this week and ahead of the talks it released 27 opposition activists.

Meanwhile, Thailand's embassy in Phnom Penh declined to comment, Reuters reported.

Thailand's immigration bureau said it had no details on the specific cases but it was typical for the UNHCR to be consulted, even though Thailand is not a party to the refugee convention.

The bureau's spokesman Achayon Kraithong said Thailand would prioritize immigration law but avoid sending people into danger

Cambodian police spokesman Chhay Kim Khoeun said the three jailed activists had violated immigration law in Thailand and Cambodia had not sought their deportation.

"They have arrest warrants against them so when they arrived, we must enact the arrest warrants," Chhay Kim Khoeun said.

"How can this be a human rights violation when living in Thailand illegally? I don't understand. Thailand enforced its law and we enforce our law," he said.