The men, in their fifties, were indicted by officials in the southern city of Lyon for the crimes of participating in a criminal conspiracy and possession of explosives, the source said.

Both are former employees of the General Directorate of External Security (DGSE), the French foreign intelligence service.

A third man was granted witness status.

The three were arrested on the island of Corsica, in the eastern French region of Haute-Savoie, and in Orleans in the country's centre, respectively.

The indictment relates to plans to kill "in the coming weeks" a political opponent, resident in Paris, of the Congolese government.

The target's identity was not divulged.