HANOI (Reuters) - Vietnam on Thursday defended its decision to revoke the citizenship of French-Vietnamese dissident Pham Minh Hoang, a former political prisoner who is accused of breaking the law and threatening state security.

The 62-year-old mathematics lecturer was told of the decision last week, drawing criticism from human rights groups. The government has not said exactly what he is accused of.

“The removal of citizenship was conducted in accordance with the provisions of Vietnamese law,” said foreign ministry spokeswoman Le Thi Thu Hang.

Hoang and the French embassy in Vietnam have been informed of the decision, she said.

Reuters was not able to contact Hoang for comment. The French embassy was not immediately available for comment.

Hoang was sentenced to three years in jail for attempted subversion in 2011 but was released after 17 months and served three years under house arrest.

The decision to revoke Hoang’s citizenship was unjustified and “marks a new low for Hanoi’s treatment of political dissidents”, U.S.-based Human Rights Watch said this week.

Despite sweeping reforms to the economy and growing openness to social change, the Communist Party retains tight media censorship and tolerates no criticism. Dozens of bloggers and activists are serving sentences for crimes against the state.