More than 400 Vietnamese drug addicts have escaped from a rehabilitation centre where they had been detained so that they could receive compulsory treatment, a local official said.

The detainees, many wielding sticks, broke free from the centre near the port city of Haiphong in northeast Vietnam late on Sunday.

"More than 400 inmates fled after breaking the door and threatened the guards of the centre with sticks," Nguyen Huy Hoang, an official from Thuy Nguyen district - where the centre is located - told the AFP news agency.

US-based Human Rights Watch has denounced the conditions in Vietnam's rehabilitation centres and a UN expert has recommended they be closed.

HRW says the treatment centres are "forced labour camps" where inmates do not receive proper healthcare and are often subjected to physical violence.

Police found some of the addicts back at their homes, while about 30 others voluntarily returned to the treatment centre.

"The police are searching for those who are still at large," in Haiphong, the third largest city in Vietnam, Hoang added.

Previous breakouts

The communist government enforces the compulsory treatment programme for the country's estimated 140,000 drug addicts.

Addicts must undergo two-year spells of "rehabilitation" in what the government describes as an effort to bring down rising rates of drug use, especially among young people.

The treatment period at the Gia Minh centre, where the breakout took place, was recently extended to three years, Hoang said.

The centre has also reduced the amount of money spent on food, prompting complaints from the addicts, he added.

In May 2010 and April 2012, detainees at similar addiction treatment centres in Haiphong also staged breakouts.

Addicts are mostly forced to report to the centres by their family or local authorities, but they are not considered criminals.