Under Vietnamese law prostitutes can be detained for between three and 18 months while drug addicts can be held for up to four years in what are officially described as rehabilitation and treatment centres, the UN said.

Detainees do not have the right to legal hearings or representation, as required under UN human rights treaties to which the country is a party, it said.

The UN "urges the government of Vietnam to revise as a matter of urgency the practice of compulsory detention for people who use drugs and (for) sex workers", it said in a position paper posted on its website.

The UN did not say how many drug users or sex workers are being held but government figures issued last year showed the communist country had about 150,000 drug addicts and 30,000 prostitutes.

Heroin is the most commonly used illicit drug in the country of about 86 million people, where drug addiction and prostitution are officially viewed as "social evils".

As in other Asian countries, many Vietnamese men use sex workers, whose services are widely available at massage parlours and elsewhere.

The UN said there is no evidence that detention prevents sex workers from later returning to their trade, and relapse rates are "reported to be very high" among former drug detainees.

"These centres for sex workers and people who use drugs do not provide either effective treatment or rehabilitation and the (UN) does not support their use," it added.

Despite its concerns, the UN noted the success of pilot methadone treatment clinics in the country.

Vietnamese officials did not immediately respond to a request for comment.

"The government has taken drastic measures to help drug users to undergo detoxification, find jobs, and re-integrate into the society," it said last year.

Agence France-Presse - September 6, 2011