HANOI (AFP) - The Vietnamese authorities have vowed to "seriously punish" a prisoner caught posting photographs of drugs alongside status updates on Facebook from a smartphone smuggled into jail, state media reported.

Nguyen Duc Hung, jailed on drugs charges, last week posted photographs of himself in a black-and-white striped prison uniform, as well as photos of him and other inmates taking drugs. The drugs were presumed to be heroin, a news website called Motthegioi reported on Sunday.

Hung made the posts "so that his wife at home could see him", the report added. He also uploaded shots of other prisoners at the Tan Lap jail 80km north-west of Hanoi, chatting on phones, showing off their tattoos and eating in their cells.

The photographs have since been taken down, the Dan Tri online newspaper reported.

The police said security at the prison, which overwhelmingly houses people serving time for drugs-related convictions, was sometimes compromised due to its location in a residential area.

"Mobile phones, and even drugs, are often thrown in from outside," police lieutenant general Cao Ngoc Oanh said, saying these kind of violations of prison security were "inevitable".

The authorities are however "determined to make the jail clean", he said according to the Motthegioi report, adding that those involved in the case "will be seriously punished".

Vietnamese jails are often overcrowded and prisoners routinely complain about poor-quality food and bad treatment.

In July last year rioting inmates at a prison in northern Vietnam briefly seized control of their jail and took the warden hostage to demand better treatment and conditions.

Family members of prisoners have told AFP that criminal - as opposed to political - prisoners have easy access to mobile phones, drugs and tobacco, if they can pay.

Facebook - which has some 25 million active users in Vietnam, according to industry figures - is the most popular social networking site in the communist country.