The Vietnamese government has arrested a prominent writer, Nguyen Vu Binh, for the second time in as many months, following a search of his home in Hanoi, according to rights groups.

The New York-based Committee to Protect Journalists said although the reasons for Mr Nguyen's arrest on Wednesday were not made public, it believed he was detained for an essay he wrote criticising border agreements between China and Vietnam.

Nguyen Vu Binh: A thorn in Hanoi's side

Human rights groups have said the government's treatment of Mr Nguyen - who was arrested briefly in late July, and reportedly subjected to frequent interrogation sessions since then - is part of a growing campaign of harassment of writers and dissidents.

In a letter to Vietnam's President, Tran Duc Luong, the CPJ urged the government to respect press freedoms.

Mr Nguyen has written several articles calling for political reform, and the CPJ said that last month he wrote an essay entitled Some Thoughts on the China-Vietnam Border Agreement, which was distributed over the internet.

Controversial deal

The land pact, signed in December 1999, more than 20 years after China and Vietnam fought a bloody border war, has been roundly criticised by many Vietnamese intellectuals who accuse Hanoi of conceding too much to its more powerful neighbour.

Mr Nguyen formerly worked on the Communist Party's official journal.

He clashed with the authorities last year when he left his job and tried to set up an opposition party.

He was among a group of dissidents detained last September in a crackdown which surprised many observers given that it coincided with the installation of a new Communist Party chief, Nong Duc Manh, widely thought to have some reformist credentials.

The CPJ also on Friday called for the release of another writer, Le Chi Quang, who was detained in February after writing an online article criticising the land and sea border agreements with China.

The group said that in August Mr Quang's mother was informed that her son would "soon" be tried on national security charges.

Despite Vietnam's constitutional guarantees of a free press, in reality dissidents take considerable risks if they speak out.