French police smashed a neo-Nazi cell accused of plotting attacks on Jewish or Muslim places of worship, judicial sources said Tuesday.

Five members of the group, who were "close in ideology to the neo-Nazi movement" were charged between September and May over the alleged plot, which was still "ill-defined", a source close to the investigation said.

"The investigation suggested they were developing an ill-defined plot to carry out an attack, likely to target a place of worship," the judicial source said.

The investigation had been ongoing for several months, leading to the arrest of five people who were planning attacks against several targets, including the annual dinner of the Representative Council of French Jewish Institutions (CRIF) and Muslim places of worship, according to reports.

Among the suspects are an assistant volunteer constable and a 15-year-old, according to reports in the French press. The arrests are believed to have taken place between September 2018 and the end of May 2019.

The five suspects had named their group 'L'Oiseau Noir' ('Black Bird) and they communicated via a forum where they discussed the possibilities of an attack, it was reported.

The investigation was opened after the September 2018 arrest of a volunteer constable whose contract was expiring in Grenoble, with investigators finding Kalashnikovs, explosives, a Glock pistol and a rifle at their home.

The investigation led them to the four other suspects, two of them minors.

Anti-terrorism investigators took over the investigation in January and charged the suspects with terror offences, including making and transporting explosive devices and being part of a terrorist conspiracy.