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...