The Eiffel tower and the Paris skyline through a small-particle haze, 13 March 2014.

Authorities in the Paris region say they will make public transport free if they have to bring in emergency restrictions on traffic because of high pollution levels caused by unseasonably warm weather.

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Nearly a third of France's 22 regions were on maximum pollution alert on Thursday for third day running.

A lack of wind, cold nights and warm days have increased dangerously small particles from traffic and heating systems in the air in areas including the Ile de France around Paris, northern and eastern France, Brittany, the Loire and Burgundy.

Paris is basking in unseasonable sunshine but that has meant a dangerous dose of particles that pushed air pollution to levels above those in other northern European capitals this week.

To encourage the use of public transport, the city council announced that green transport, such as its Velib bike-share and the Autolib electric car fleet, would be free on Thursday, as would residential parking.

And the Socialist chairman of the Ile de France regional council, Jean-Paul Huchon, said it was preparing drastic restrictions on the use of highly polluting vehicles and is already tightening speed limits and banning heavy goods vehicles from the city.

If the tough measures come into effect, the region is ready to make travel on public transport free while they are in operation, he said.

The French capital is experiencing unseasonably warm weather in March, with weather forecasters saying Paris would enjoy sunny skies and an average temperature of 19°C.

Last year in March, snow covered the country and temperatures were below zero in Paris (-1°C) on 12 March 2013.

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