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.
Just days after an , Australian citizen Chau Van Kham is said to have been detained by Vietnamese authorities.
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 the authorities in Hanoi consider terrorist organisations.
The authorities have accused the Viet Tan of "instigating violence", a charge the group rejects.
Fifteen members of a separate group - the Provisional National Government of Vietnam - were also branded terrorists and convicted over a 2017 plot to attack Ho Chi Minh City's airport with petrol bombs.
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."