French President Nicolas Sarkozy, several of his ministers and other politicians have received envelopes containing death threats and bullets, a judicial source said on Tuesday.

The politicians all received brown paper envelopes within the past two weeks containing a 9mm cartridge and identical letters from an unknown sender with threats like "you are all dead men walking", the source said.

The anti-terrorist unit of the Paris prosecutors' office as well as anti-terrorist police in Paris and the southwestern district of Herault have been put in charge of investigating the threats.

There was no comment from Sarkozy's office or any of the ministers' offices.

The latest person to receive one of the letters was former Prime Minister Alain Juppe, now mayor of Bordeaux in the southwest.

"I'm not worried. It is up to the police to analyse the evidence and take necessary measures," Juppe told reporters.

Among other threats, the letters say: "You think you have control of your lives, well no, your lives and your families' lives are in our hands."

It was not clear whether the author was making any specific demand or defending any stated cause.

The judicial source in Paris said the offices of Sarkozy, Justice Minister Rachida Dati and Interior Minister Michele Alliot-Marie had received letters. Other recipients included legislators from Sarkozy's UMP party in southwestern France.

Bordeaux chief prosecutor Claude Laplaud told reporters there was no clear lead so far. He said investigators were considering the possibility that the letters could be the work of a mentally unstable person.

Juppe said he had lodged a formal complaint to the police because it was normal procedure to do so, but was not asking for extra protection.

"I am not going to change my plans. If you start reading anonymous letters when you're in politics, you'll never finish," he said.