Vietnam has extended the detention of the human rights lawyer, Nguyen Van Dai, whose arrest last year signalled the beginning of a wave of repression against government critics.
Mr Dai and his assistant, Le Thu Ha, have been held in isolation since December, denied access even to family members and lawyers.
The authorities added another four months to their detention without giving any suggestion of when they would be brought to court.
The office of the UN High Commissioner for Human Rights said it was concerned about the treatment of the two activists and called for their immediate release.
“Authorities have refused to provide information about their whereabouts and health conditions,” said the Commissioner’s Southeast Asia office in a post on its Facebook page.
It expressed fears that “prolonged incommunicado detention may facilitate the perpetration of torture and can in itself constitute a form of cruel, inhuman or degrading treatment or even torture.”
Mr Dai was arrested as he prepared to meet members of a European Union delegation, who were in Vietnam for a human rights dialogue with the government. The arrest was seen as a show of disdain by the Vietnamese authorities for foreign pressure over political freedoms.
Mr Dai is seen as leading member of the campaign for political pluralism and respect for human rights. He resumed his activities last year after serving four years in prison and four years on probation after the crushing of an earlier reform movement.
The OHCHR statement urged the government to abide by its legal obligation to prevent torture and the ill-treatment of prisoners under the Convention against Torture and the International Covenant on Political and Civil Rights (ICCPR).
“Our Office reiterates our calls on the Vietnamese Government to stop using vague and broad provisions related to ‘national security’ to silence human rights defenders, activists and bloggers.”
It said the use of Article 88 of the penal code, which is used to imprison government critics for spreading propaganda against the state, was not in line with international standards under the two conventions.
The office also expressed concern about the prison sentences handed down this week to two cousins, Nguyen Huu Quoc Duy and Nguyen Huu Thien An, for posting Facebook messages attacking government policy.