Striking workers who have shuttered over half of France's oil refining capacity are threatening to expand their walkout to the rest of the country's refineries, raising the risk of gas shortages ahead of key regional elecions.

Workers at all six of Total SA's French refineries have been on strike for five days, and their colleagues at ExxonMobil Corp.-owned Esso France's two refineries are set to begin their own walkout Tuesday.

Together, Total and Esso refineries account for about 70 percent of France's refining capacity.

Workers at France's fourth-largest refinery, British-owned chemicals company INEOS, were meeting Monday to vote on whether they too would join the widening strike, according to Charles de Launay, a representative of the CGT union.

Two more refineries, owned by Switzerland's Petroplus, are scheduled to take a strike vote on Wednesday.

CGT strike coordinator Charles Foulard has predicted that France will reach "borderline shortage" in the coming week. The French oil refineries' body, UFIP, said this weekend that France has at least 10 days' worth of fuel stocks.

Total says it has fuel stocked away that it can draw on, though it has declined to say how many days that could last.

The strike was sparked by workers' concerns about the future of a refinery in Dunkirk, in northern France, raising fears about possible gasoline shortages. Besides the refineries, the strike has also spread to six of Total's 31 fuel depots.