A strike over new labour laws has spread to all of France's eight oil refineries, the CGT union says, in an escalating dispute with the government.

An estimated 20% of petrol stations have either run dry or are low on supplies.

Clashes broke out at one refinery early on Tuesday when police broke up a picket at Fos-sur-Mer in Marseille.

Prime Minister Manuel Valls insisted the labour laws would stand, and that further pickets would be broken up.

"That's enough. It's unbearable to see this sort of thing," he told French radio. "The CGT will come up against an extremely firm response from the government. We'll carry on clearing sites blocked by this organisation."

The union is aiming to cut output by half at the refineries and wants strikes to take place on train networks too, in an attempt to reverse labour laws that make it easier for companies to hire and fire staff.

The new laws were forced through parliament without a vote earlier this month, after the government watered down its proposals.

French labour reform bill - main points

• The 35-hour week remains in place, but as an average. Firms can negotiate with local trade unions on more or fewer hours from week to week, up to a maximum of 46 hours
• Firms are given greater freedom to reduce pay
• The law eases conditions for laying off workers, strongly regulated in France. It is hoped companies will take on more people if they know they can shed jobs in case of a downturn
• Employers given more leeway to negotiate holidays and special leave, such as maternity or for getting married. These are currently also heavily regulated