French police launched raids across the country early on Monday targeting an alleged network sending jihadist fighters to Syria, according to a police source.

Elite and anti-terror police units descended on around a dozen targets, mostly in the southern region of Toulouse, but also around Paris and in the northern region of Normandy, the source said on condition of anonymity.

It was not immediately clear how many people were arrested.

In recent months France, which has Europe’s largest Muslim population, has been facing the fact that hundreds of its citizens have openly joined jihadist groups in Iraq and Syria and have even called for attacks on their homeland.

Almost 1,000 French nationals from a wide range of backgrounds are estimated to have left to join jihadists in Iraq and Syria, including some 400 thought to be currently fighting on the ground and almost 50 who were killed.

According to figured published in the le Monde newspaper in November, about a quarter of those who left to join the jihadists were converts to Islam, with many coming from everyday French backgrounds.