France plans to ban citizens suspected of links to armed groups from leaving the country in a new bill aimed at strengthening "anti-terrorist" legislation.
The draft bill unveiled on Wednesday would allow authorities to stop French nationals from travelling if they were suspected of having links to banned groups.
It would also allow investigators to question and charge people, known as "lone wolves", with individual plans to commit violence.
The government says it is concerned about preventing attacks by individuals returning from the civil war in Syria.
About 600 French nationals are in Syria to fight with rebel groups or are planning to go there, according to the Interior Ministry.
The plan is to be presented to the parliament in the coming weeks with an aim for a vote by the end of the year.
Teenage sisters 'in Syria'
Meanwhile, in the UK, authorities say they believe that missing 16-year-old twin sisters who disappeared from their home in Manchester have travelled to Syria.
The girls left the UK on June 26 and flew to Turkey, where police believe they crossed the border to Syria.
Tony Mole, a detective chief superintendent, said on Wednesday the sisters had put themselves in danger.
But he said police did not know why they travelled there or who they were with.
British officials say several hundred Britons have travelled to Syria to support rebels fighting the government of Bashar al-Assad.