HANOI (AFP, REUTERS) - A Vietnamese-American man is being held in Vietnam for attempting to overthrow the state, his family and a US lawmaker said, calling for his quick release from the communist country that routinely jails its critics.

Mr Michael Nguyen is the second US citizen arrested in Vietnam in recent weeks for allegedly brushing up against politics in a country where activism of any sort is quickly stamped out.

The 54-year-old flew to Vietnam on June 27 to visit friends while his wife, Helen, stayed in California with the couple's four daughters, Mr Nguyen's brother-in-law Mark Roberts said. Mrs Nguyen, who works as a nurse, was last in contact with her husband on July 5.

US Consulate officials in Saigon confirmed to the family this week he was detained on July 7 under Article 109 of the criminal code - "overthrowing the people's administration" - and is being held in a Ho Chi Minh City prison with no formal charges.

"Michael is under investigation for Article 109," Mr Roberts told reporters in California on Thursday (Aug 2), according to a transcript of the briefing provided to AFP.

California congressman Mimi Walters confirmed in a statement that Mr Nguyen has been detained and she is trying to secure his release.

The maximum sentence under Article 109 is death.

Vietnam's Foreign Ministry spokesman told reporters on Thursday that she would ask "relevant authorities" about the case, while the US Embassy spokesman Pope Thrower said "we are aware of media reports that a US citizen is being detained in Vietnam", declining to elaborate.

Mr Nguyen's friend Le My Hanh told AFP he went missing while travelling in southern Vietnam with three activists, but his brother-in-law said he was not involved in politics.

"We're asking the government of Vietnam to please release him," Mr Roberts told AFP, adding that the family is devastated over his arrest.

Mr Nguyen, who was born in Vietnam and has lived in the United States since he was a child, was not known to be politically active and spends his time in California raising four daughters and running a printing business, his family said.

He is the second American citizen detained in Vietnam this year after Yale graduate Will Nguyen was arrested for joining protests in Ho Chi Minh City in June.

He was deported last month after being convicted for causing public disorder in a case raised by US Secretary of State Mike Pompeo.

Vietnam's communist government routinely jails lawyers, activists and journalists it deems critical, but a conservative leadership in charge since 2016 is accused of intensifying the crackdown.

Scores are behind bars and the campaign has sent a chill throughout the activist community, forcing several into hiding or to self-censor online.