France's interior minister said on Tuesday that police had arrested three suspected Jihadists whom a security expert said were part of a group sending Islamist fighters to Syria.

Socialist President Francois Hollande has made clamping down on violent cells and self-radicalized "lone-wolf" operators planning domestic attacks a top priority since an Al-Qaida-inspired gunman shot dead seven people in March 2012.

France has been on heightened security alert since January, when it intervened in Mali to repel Al-Qaida-linked rebels who had seized control of the north of the former French colony.

"These are individuals already known to us, notably for threats to republican institutions and our values on the Internet," Interior Minister Manuel Valls told reporters following the arrests in southern France.

"This certainly shows the threat is still present."

France is concerned that some 100 to 200 French citizens who have left for Syria to fight against President Bashar Assad could return and plot attacks against French interests.

Valls has said at least 30 such fighters have returned and suggested the three arrested were part of such a group.

"We need very powerful action to attack the phenomenon linked to terrorism and to these channels that prepare individuals to fight in Syria in Jihadist groups that call themselves Al-Qaida and are particularly dangerous," he said.

The head of the International Terrorism Observatory think tank, Roland Jacquard, told Reuters Television the group appeared to be sending fighters abroad, likely to Syria.

The arrests came a day after police rounded up six other people, aged 22 to 38, in the Paris area. All were known to police for organized crime offences.

Valls 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.

Jacquard described the six as "gangster terrorists" who had been planning a bank heist.

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.

Many small groups

Also on Tuesday, police in Germany raided several flats of suspected Islamists around Stuttgart, Bavaria and Saxony, but there were no arrests. The federal prosecutors' office said the suspects may have been planning attacks using remote-controlled model airplanes.

Jacquard said the two French groups were a good example of small autonomous cells, many of which have no visible ties to known terror groups, that operate in France and Europe.

The police did not give the nationalities of those arrested. Terrorism experts say those attracted to violent Islam are often young men from tough neighborhoods around big cities who are second- or third-generation French born of Muslim immigrants.

France's worst terror attack in nearly 30 years occurred in March 2012 when French-born Mohamed Merah shot dead a rabbi, three Jewish children, and two soldiers in and around the southern city of Toulouse.

More recently, a Muslim convert suspected of stabbing a French soldier in Paris in a religiously motivated attack was placed under formal investigation last month.

French authorities have carried out 21 terrorism-related operations so far in 2013 and arrested 48 people. Seventeen have been jailed, according to the interior ministry.