PARIS, March 17: President Jacques Chirac called on France to remain vigilant on Wednesday even as Paris said a threat from a shadowy Muslim group to sow "terror and remorse" did not bear classic extremist hallmarks.

French prosecutors have launched a criminal probe into the hitherto unknown Movsar Barayev Commando, which vowed in a letter to Paris-based newspapers to punish France for banning Muslim headscarves in state schools.

"The terrorist threat demands continual vigilance. This vigilance must be greater in current circumstances," Mr Chirac told his weekly cabinet meeting, according to his spokeswoman.

Mr Chirac insisted the threat should not have any impact on the first round of French regional elections being held on Sunday. "Democracies must react calmly, responsibly and be careful to remain true to themselves. They should not allow themselves to be swayed," he said.

Interior Minister Nicolas Sarkozy said the letter did not contain characteristics of threats by Muslim groups received in the past. "We are studying this letter and the specialists are of the opinion that its phraseology does not match the usual style used by extremists for this type of letter," he told reporters after the cabinet meeting.

"That does not mean we are ignoring it. Our duty is to take everything into account," he added. Spain was hit by devastating bombings last week ahead of its general elections on Sunday. France has tightened security at rail stations, airports and other public places in the wake of the Madrid train bombings.

The letter said France had been originally excluded from a list of infidels "because of your opposition to the unjust aggression by the crusaders in Iraq", a reference to Paris's forceful diplomatic campaign against the US-led invasion.

But a law banning Muslim headscarves in state schools from September had earned France everlasting enmity and the group would ask "Allah to sow terror in the hearts of the French".

French media quoted police as saying the letter, unusually well structured and containing only minor grammatical errors in French, was written by someone with solid knowledge of fundamentalism.

Police noted it carried none of the usual quotations from the holy Quran or markings that have accompanied previous threats to France from Muslim groups. The group appears to be named after the Chechen guerilla chief who died leading a squad of fighters on a hostage-taking operation in a Moscow theatre 18 months ago.

France has detained several suspected militants alleged to have trained in Afghanistan or Chechnya with groups linked to the Al Qaeda. The trial of three such suspects accused of links to terror groups started in Paris on Wednesday.

Frenchmen David Courtailler and Ahmed Laidouni and Algerian Mohamed Baadache are suspected of having visited camps in Pakistan and Afghanistan used to train radicals.

David Courtailler, 28, a convert to Islam, is said by police to have links with Jamal Zougam, a key suspect detained by Spanish police in the Madrid bombings. He pleads innocent. "He is going to ask for the case to be dismissed," lawyer Philibert Lepy said. "There's nothing against him in the case file that proves he intended to commit a terrorist act." -Reuters