PARIS - A French physicist of Algerian descent, arrested on suspicion of collaborating with Islamic militants, made only vague references to attacks in emails intercepted by U.S. intelligence services, a magistrate told Reuters on Sunday.

The 32-year-old, who worked at a nuclear research institute, was in contact with people close to Al Qaeda in the Islamic Maghreb (AQIM), a mainly Algerian Islamic militant network that associates itself with the group led by Osama bin Laden.

In his exchanges with the AQIM, he spoke of plans for attacks in general, but we're not talking on an operational level, it was not a case of means or dates, said the magistrate, who asked not to be named.

He had proposed targets, but it was inconsistent, the source said, adding that there had been no talk of a nuclear attack.

The suspect, who worked at the European Organisation for Nuclear Research (CERN) in Switzerland, was still under police custody on Sunday, a spokeswoman for the Paris prosecutor's office said.

He is due to appear before an anti-terrorist magistrate on Monday. His younger brother, who was arrested with him on Thursday in Vienne, southeastern France, has been released without charge, the spokeswoman said.

The suspect had been under surveillance for a year as part of a wider investigation into the recruitment of fighters in Afghanistan. U.S. intelligence services had alerted French anti-terrorism authorities after intercepting his email exchanges.

It was only after the dossier was handed last week to a new examining magistrate, Christophe Teissier, that it was decided to arrest the suspect, the source said.

CERN, a centre for research on particle physics, said on Friday that the arrested man's work did not bring him into contact with anything that could be used for terrorism.

He had been working on an experiment in particle physics as a contractor since 2003, it said.

Afghanistan Algeria