PARIS—France is trying to prevent French nationals from traveling to Syria to join jihadist groups fighting the regime of Syrian President Bashar al-Assad, underscoring concerns across Europe that Syria has become an incubator for future terrorist attacks on European soil.

French President François Hollande said his government will adopt a series of measures Wednesday aimed at stemming the flow of French residents who join al Qaeda-linked jihadist groups in Syria. Paris is also sharpening its crackdown on French nationals who return home from Syria after taking part in armed conflicts there.

The government will deploy "all kinds of measures to dissuade, to stop, to punish those who would be tempted to go fighting in places where they have no place," Mr. Hollande told reporters on Tuesday.

More than 500 French nationals are fighting in Syria, according to French officials who worry the conflict has become a recruitment tool and training ground for jihadist groups that aim to radicalize people who carry European passports.

Under the new measures, French authorities will be required to monitor websites affiliated with jihadists groups that recruit new members from inside France and then arrange for them to travel to Turkey and other countries that border Syria before entering the conflict area.