French police have arrested a 15-year-old boy suspected of planning an attack, investigators said Sunday, as Prime Minister Manuel Valls warned that the country faced a threat from 15,000 homegrown radicals. 

The teenager was arrested in eastern Paris on Saturday and had been under house arrest since April for suspected links to Islamic extremists.

The police suspect him of plotting “in response to calls from Syria to attack France,” one of the sources said, just days after a separate plot to blow up a car packed with gas canisters was allegedly foiled in central Paris.

Investigators are looking into calls made by a French member of the Islamic State group, Rachid Kassim, for supporters of the group to strike French targets.

The source said the 15-year-old had been in touch with Kassim using the encrypted messaging app Telegram.

Kassim has regularly appeared in IS propaganda videos calling for attacks on French targets.

He has been linked to at least one of the two teen jihadists who executed an elderly priest in a Normandy church in July.

Appearing on French television on Sunday morning, Prime Minister Manuel Valls said that around 15,000 people were known to police in France as having been radicalized, up from a previous estimate of 10,000.

He said around 700 jihadists from France were fighting in Syria and Iraq. 

Interior Minister Bernard Cazeneuve said police had arrested 293 people this year for “links to terrorist networks.

Agence France Presse