A French judge has opened criminal proceedings against four men suspected of links with Islamist militant networks in west Africa, BBC News reported.

The four, who were detained in the Paris area earlier this week, are being investigated for possible "association with a terrorist enterprise". Three are French and one is from Mali.

The government has warned of possible militant attacks in France in the wake of the French operation in Mali.

Two of the four men are to remain in preventive detention. The two others were released, but must report regularly to authorities.

French officials have said they were arrested in connection with the case of a French national arrested in Niger while trying to join Jihadist groups in neighboring Mali last year.

The man was extradited to France, where criminal proceedings were launched against him in August.

Last week French Interior Minister Manuel Valls said: "We have to continue dismantling these networks that want to either commit attacks on our soil or take individuals overseas to carry out jihad."

France increased its domestic terror threat level after French forces went into action in Mali on 11 January to push back militants who had captured Timbuktu and other parts of northern Mali.