Around 100 militants who fought with rebel groups in Syria and Iraq have returned to France, requiring "massive" resources for surveillance and other security measures to prevent attacks, a French lawmaker said on Friday.Thousands of Western volunteers have travelled to Syria and Iraq to join Islamist fighting groups, notably Islamic State. The exodus has raised fears in Europe and the United States of attacks by returning fighters.Of an estimated 1,000 volunteers who left from France - the top source of Western volunteers for the Islamist jihad in the region - around 100 have returned and are currently in the country, Socialist lawmaker Sebastien Pietrasanta told Reuters."Some of them are in jail, others are under judicial surveillance," said Pietrasanta. "We have material evidence showing that a number of those who have returned from Syria could potentially have gone ahead to commit attacks."Pietrasanta is the chief spokesman for an anti-terrorism law to be debated in parliament on Monday that tightens surveillance of potential jihadi volunteers and restricts the movements of returned fighters, who could be stripped of their passports and identity papers.Surveillance of returned fighters was taking place on a "massive scale", said Pietrasanta."When you figure that it takes about 20 security agents to keep watch on one person, you get a sense of the challenge facing our security services," he said, adding that 52 returned militants were currently in jail.