France is stepping up security in airports and train stations and building up its counterintelligence ranks in response to threats in an audiotape attributed to Usama bin Laden.

"France was notably singled out" in the audio message aired Tuesday on the Arabic television network Al-Jazeera, Interior Minister Nicolas Sarkozy said Thursday.

In the tape, a voice attributed to bin Laden threatens six U.S. allies: Britain, France, Italy, Germany, Canada and Australia.

There was no sign of increased security in Britain, Germany or Canada, which all said they were taking no additional security measures. However, Britain stepped up checks at ports last weekend as a result of other recent warnings.

Sarkozy said France was undertaking an "ambitious and important civil defense action" to counter terrorism. "The threat exists. Each of us is aware," he told senators during question time.

"We have no precise elements" regarding threats to France, the minister said, adding the new measures would be instituted because "we take all risks seriously."

Adding 200 agents to the ranks of the DST counterintelligence service is among the measures, Sarkozy said. Security at airports and train stations will be stepped up shortly, he said without elaborating.

Last week, Justice Minister Dominique Perben said a new anti-terrorism magistrate would be added to investigate cases, increasing the number of such judges to five.

Anti-terrorism judges order arrests or sweeps of suspects in their investigations. In one such development, police announced Thursday that the head of a suburban Paris mosque and two acquaintances were detained for questioning.

Thami Raiji, a 52-year-old Moroccan who is the imam, or head, of the mosque in Puteaux, west of Paris, was detained on Wednesday with the two others, both bearing the first name of Rachid, police said.