PARIS (AFP) - French prosecutors have opened a preliminary probe after an environmental group filed a criminal complaint over this month's spike in air pollution, a judicial source said on Monday.

The complaint for "endangering others" was filed by green group Ecologie Sans Frontiere amid intense pollution this month that saw particulates in the air exceed safe levels for five straight days in Paris.

Paris was enveloped in a murky haze, cars with even number plates were banned from city streets and public transport was provided for free as officials sought to improve air quality.

Ecologie Sans Frontiere said it was the first time such a criminal complaint had been filed.

The complaint does not specify who could be held responsible for the pollution, but the group's vice-president Nadir Saifi said investigators could eventually target car manufacturers or certain government offices.