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France has begun importing electricity as the wave of strikes against pension reforms takes hold of energy supplies.

The news emerged as President Nicolas Sarkozy warned that the economy and jobs could be hit if the disruption did not end quickly.

In one hour on Wednesday, 5,990 megawatts were imported, equivalent to the output of six nuclear reactors.

At least 12 of France's 58 reactors are shut for maintenance but the unions say production has been cut at four others.

The CGT union said striking workers had lowered output at the Cattenom nuclear plant in north eastern France.

It also claimed that strikers in the South West had cut power to 15 town halls controlled by Mr Sarkozy's UMP party.

France's other energy supplies have already been severely disrupted by industrial action which has left all the country's 12 oil refineries blockaded for more than a week and a quarter of its petrol stations empty. MPs were told on Wednesday that 3,190 had run dry.

Police reopened three fuel depots in western France overnight, after ministers ordered blockades be lifted.

Striking gas workers were also reported to have stopped gas from being injected into the country's network from three out of 12 storage sites.

President Sarkozy issued a statement insisting that the disruption could not continue: "If it is not stopped quickly, this disorder which is aimed at paralysing the country could have consequences for jobs by damaging the normal running of economic activity."

He said he would press ahead with unpopular plans to raise the retirement age from 60 to 62 and the full state pension age from 65 to 67.

The upper house of the French legislature is due to vote on raising the retirement age on Thursday although opponents are seeking to delay the bill.

Interior Minister Brice Hortefeux authorised use of the paramilitary police to break blockades at fuel depots. He said he respected the right to protest, but that did not include the right to block workers or to commit pillage or violence.

The 91�ȱ�'s Christian Fraser said this force was the equivalent of a Swat team whose normal duties include hostage rescue.

In the early hours of Wednesday, riot police lifted the blockade at three fuel depots, in Donges, La Rochelle and Le Mans. However, strikers reimposed their blockade at Donges.

In the south of the country, unions blocked the fuel depot at Trapil, which supplies civilian and military airports in the region, but were later said to have left the site of their own accord.

Our correspondent said the unions and the government were engaging in a game of cat-and-mouse over the fuel blockades.

There was also further sporadic violence in areas affected by disturbances earlier in the week.

In the Paris suburb of Nanterre, youths smashed shop windows and threw stones while in Lyon there were skirmishes between rioters and police.

Mr Hortefeux, on a visit to Lyon, condemned the violence, saying it was "unacceptable" that more than 60 police officers had been injured.

Airports blocked

Transport workers continued their protests and the national rail operator, the SNCF, said one in three high-speed TGV trains had been cancelled.

About 300 striking workers staged a protest at France's main airport, Roissy-Charles de Gaulle in Paris, blocking access to the main terminal building. They sang the national anthem before pushing through a police barricade and marching through the building, blowing whistles and waving flags.

Protesters also blocked the main road leading to one of two terminals at Orly airport in Paris, then blocked the road to the other terminal, according to the Paris airport authority.

President Sarkozy's poll ratings appear to have dropped even further as he tries to tackle the wave of protests.

One poll for BVA conducted on 15 and 16 October suggested his approval rating was down to 30%, the lowest for three years.

The number of French with either a negative or very negative opinion of their president rose five points from September to 69%.