The ban covers several books being sold illegally, including the diary of a leading dissident, Tran Do.

Underground publications thrive despite government moves

He is a retired general and former senior official in the Communist Party who has been calling for reform.

Tran Do has in the past complained of being harassed over his diary and his suggestions for modernisation.

Another publication that's been targeted is called Meditation and Aspiration, written by a geophysicist, Nguyen Thanh Giang.

He has also complained of surveillance and of being kept under virtual house arrest.

Vietnam has a flourishing trade in unofficial publications, which are produced on computers and photocopied for underground circulation and sale on the streets.

The Communist Party has little tolerance for unapproved activities.

Last September, two dissidents in Hanoi were detained after asking for permission to set up an independent anti-corruption body.

The authorities in Vietnam strongly deny reports from overseas human rights groups about the suppression of dissent and restrictions on religious freedom.

The party says it does not hold political prisoners, but only those who break the law and threaten national security.