A 27-year-old Swiss man arrested in France was in contact with a 14-year-old French boy who was "about to carry out the attack," one of the sources said.
The teenager was arrested in the Paris region on June 20th and charged by an anti-terror judge, the sources said.
A photo of the boy holding a paper vowing his allegiance to the Islamic State group was found on social media, they added.
The Swiss man was "particularly active on social networks... and he had contacts with individuals living in France with whom he notably discussed violent actions," a judicial source said.
Nine suspects were arrested in southern France and the Paris region and were aged between 18 and 65, the sources said, adding that most were known to the authorities.
The suspects exchanged "disturbing remarks" on the encrypted messaging app Telegram, they said, adding that investigators were seeking to "determine what they were plotting".
A 23-year-old Colombian woman was arrested in Switzerland, while those arrested in France included two brothers who were known to have been radicalized, the sources said.