French police on Tuesday detained three men and two women as part of a probe into Mehdi Nemmouche, who is suspected of killing four people at Brussels' Jewish Museum in May, a judicial source said.

Nemmouche, 29, a Frenchman of Algerian origin who spent more than a year fighting with Islamic extremists in Syria, was arrested in the southern city of Marseille shortly after an Israeli couple, a French woman and a Belgian were shot dead at the museum in central Brussels.

He was on a bus from Brussels carrying a revolver and Kalashnikov rifle in his luggage, and has since been charged with "murder in a terrorist context".

Those arrested on Tuesday include people Nemmouche may have met while in detention, said a source close to the case. Further details about them were not known.