BANGKOK (Reuters) - Three Thai policemen approached Vietnamese refugee Nguyen Van Chung at his home in Bangkok in January and asked him whether he was in touch with another Vietnamese man, Truong Duy Nhat, who had fled to Thailand.

FILE PHOTO: Prime Minister of Thailand Prayut Chan-o-cha shakes hands with Vietnamese Prime Minister Nguyen Xuan Phuc at the National Convention Center in Hanoi, Vietnam March 31, 2018. Minh Hoang/Pool via REUTERS/File Photo

Chung said no, he had never met Nhat, a writer and critic of Vietnam’s communist government who previously had spent two years in prison for “abusing democratic freedoms”. He only knew of Nhat from his Facebook posts.

But during a subsequent interrogation, Chung was surprised to notice a man who seemed to be a Vietnamese official, and Thai police then confirmed he was indeed from Vietnam.

“Somehow, discreetly, police of Vietnam and Thailand worked together and knew everything,” Chung told Reuters from a third country, where he fled soon after.

The encounter was telling because Nhat, the writer the police were looking for, disappeared two days later from a Bangkok shopping mall.

He resurfaced in a Vietnamese jail.

U.N. envoys, in letters to Vietnam and Thailand, raised suspicion of an “enforced disappearance” and expressed “grave concern”. Both Thailand and Vietnam declined to comment.

Nhat’s case is not the only one in recent months.

As leaders of the 10-member Association of Southeast Asian Nations (ASEAN) meet this weekend in Bangkok, human rights campaigners decried what they called increased cooperation in the forced return of refugees and asylum-seekers.

Since last year, there have been at least eight cases of Southeast Asian governments being accused of either officially arresting, or cooperating in the abduction of, political refugees from fellow ASEAN countries.

“A number of countries in the region are trading off political dissidents and individuals fleeing persecution as part of an unholy alliance to shore up each other’s regime,” said Nicholas Bequelin, Amnesty International’s director for East and Southeast Asia.

‘EXTREMELY WORRYING’

Authorities in Vietnam, Cambodia, Malaysia and Thailand have all been accused of detaining and returning critics of neighboring governments, in some cases even when they had political refugee status with the United Nations.

“The growing trend of Southeast Asian governments returning dissidents to neighboring states where they could be at risk is extremely worrying,” said Charles Santiago, a Malaysian lawmaker and chairman of the ASEAN Parliamentarians for Human Rights.

Thailand, which is hosting this weekend’s ASEAN meeting, declined to comment on rights groups’ complaints.

“We don’t have information on any of these cases,” said a Thai foreign ministry spokeswoman.

Thailand was once considered a haven for activists fleeing repression from authoritarian governments.

But since a military coup in 2014, Thailand has requested return of its own political opponents from neighboring countries - and has also obliged similar requests from them, critics say.

Last month, Malaysia arrested and sent home a Thai anti-monarchy campaigner after she registered as an asylum-seeker with the U.N. refugee agency.

The woman, Praphan Pipithnamporn, is awaiting trial on sedition and organized crime charges in Thailand.

Malaysian leader Mahathir Mohamad defended the extradition, saying his country was “a good neighbor”.

QUID PRO QUO?

Last year, Thai authorities arrested two Cambodians and sent them home.

Sam Sokha, a labor activist, had thrown a shoe at a poster depicting Cambodian Prime Minister Hun Sen. She is serving a two-year prison sentence for “insulting a public official”.

Another Cambodian, Rath Rott Mony, was arrested in Bangkok in December and sent back. He faces three years in prison for “incitement to discriminate” for his role in a documentary on child prostitution. The verdict is due on June 26.

While some activists are extradited through legal channels, there have also been reports of extrajudicial abductions, such as Nhat’s.

U.N. envoys in letters to Vietnam and Thailand dated April 18 were blunt about their suspicions, saying they believed Thai immigration and police officials, a “representatives from a Thai ministry” and Vietnamese military intelligence officers from Hanoi were involved in the suspected abduction of Nhat.

In February, three exiled Thai anti-junta activists who had been living in Laos – Siam Theerawut, Chucheep Chivasut and Kritsana Thapthai – disappeared.

They were arrested by Vietnamese authorities while in transit and handed over to Thailand, according to U.S.-based Thai Alliance for Human Rights.

Neither Vietnamese nor Thai authorities would comment on the report, which Reuters could not verify.

Still, the timing of the activists’ disappearances raised suspicion, Amnesty’s Bequelin said.

“The chain of events in Nhat’s case suggests a possible quid pro quo exchange between Thailand and Vietnam,” he said.

The bodies of two Thai activists who fled to Laos were found on the Thai side of the Mekong River in January, cut open and weighed down with chunks of concrete. No one knows - or is saying - what happened to them.