A prominent Vietnamese-Australian pro-democracy activist has been detained by security services on his return to Ho Chi Minh city, colleagues in Australia told AFP on Friday.
The detention of Chau Van Kham comes just days after an Australian blogger and author was arrested in China.
The retired businessman was on a "fact-finding" mission to the country, entering via Cambodia, according to colleague Phong Nguyen. Both men are members of the Viet Tan, one of a number of anti-government groups that Hanoi considers terrorist organisations.
Vietnamese authorities have accused the Viet Tan of "instigating violence", a charge the group rejects.
Colleagues said they did not know where Chau was being held, or whether the Australian government had been granted consular access.
It is believed he was detained on January 13. His wife and immediate family remain in Sydney.
"At the moment we don't know where he is," said Phong Nguyen. "He was aware of the risk, but he was determined to go on this fact-finding trip."
The visit was designed to assess human rights in the country, Phong Nguyen added. "We are mindful that the Vietnamese police have a history of framing peaceful activists with fabricated charges."
Vietnam is a one-party state, with severe restrictions on any criticism of the ruling Communist Party.
Freedom House -- a rights watchdog -- ranks the country as "not free" and Human Rights Watch has accused the government of recently "intensifying" a crackdown.
"During 2018, the Vietnamese authorities sought to break up several key dissident networks," HRW said in a recent overview.
"At least 42 people were convicted for expressing opinions critical of the government, peacefully participating in public protests, or joining pro-democracy groups.
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