PARIS (Reuters, AFP) - Four men were arrested on Tuesday by special police forces in the southern French town of Lunel in an action aimed at breaking up extremist networks, a police source said.

Two of them were returnees from Syria, the source said.

The town of Lunel, east of Montpellier in southern France, has attracted attention after local media reports that as many as 20 residents have sought to travel to Syria to fight alongside extremists. Six of them, aged 18 to 30, have been killed since October.

Crack French security forces launched the operation at 6:00 am local time (1pm Singapore time) in a building in the centre of the town, according to witnesses that spoke to AFP.

"Several unmarked cars drew up. Masked men got out and smashed in the doors to the apartments in the building," said one resident of the block, who said they had threatened him. "They put a gun to my head ... in the end, they arrested my neighbour above me, Said," said this witness.

Another witness said authorities had taken away his brother. "They flattened me, got me on the floor, hit me. Then they took away my brother," this man said.

The raid was still ongoing early Tuesday, a security source said.

France has stepped up its fight to break up extremist recruitment cells after 17 people were killed in three days of violence by Islamist militants in Paris earlier in January. The attacks, one at the headquarters of satirical newspaper Charlie Hebdo and the other at a Jewish supermarket, were launched by men who had connections to extremist recruiters. All three were killed as security forces closed in on them.

Since then, police in Belgium, France and Germany have interrogated dozens of suspects, and in Paris last week French prosecutors asked for formal investigations of four men suspected helping prepare the Jan 7-9 attacks in Paris.