French police have detained three suspects, adding to the previous six detained, according to the country's interior minister.
French security experts said on Tuesday the three were part of a group sending Sunni fighters to Syria.
"These are individuals already known to us, notably for threats to republican institutions and our values on the internet," Manuel Valls, the interior minister, said.
"This certainly shows the threat is still present."
President Francois Hollande has made clamping down on violent cells and self-radicalised operators planning domestic attacks a top priority since an al Qaeda-inspired man killed seven people in March 2012.
France has been on heightened security alert since January, when it intervened in Mali to repel al Qaeda-linked rebels who had seized control of the north of the former French colony.
France is concerned that up to 100 to 200 French citizens who have left for Syria to fight against President Bashar al-Assad could return and plot attacks against French interests.
Valls has said at least 30 such fighters have returned and suggested that the three arrested were part of such a group.
He called the six "particularly dangerous" and said they had a "willingness to engage in terrorism".
They were suspected of an armed robbery a few months ago in the Paris area, he said.
Under French law, "terror suspects" can be held for up to 96 hours before judges decide whether to put them under formal investigation, a step that can lead to a trial.
Meanwhile, in Germany police have raided nine flats of several suspected fighters in Stuttgart, Bavaria and Saxony.
There were no arrests following Tuesday's raids.
The federal prosecutors' office said the suspects may have been planning attacks using remote-controlled model airplanes.
German prosecutors continue investigating two men who are suspected of planning what they call "extremist attacks" amid raids.
Both are originally Tunisian.