France has begun using its reserve fuel supply to deal with petrol shortages caused by workers blockading oil refineries over labour reforms.
The General Confederation of Labour (CGT) said striking workers are preventing trucks from being able to enter to load or unload.
The union is disgruntled over the government's planned labour reforms, which it says would make it easier for employers to hire and fire workers.
The head of UFIP, the group overseeing the nation's petroleum industry, told French radio the government has approved the use of fuel stocks.
Francis Duseux said: "For the past two days, since there have been operational problems at the refineries, blockades of depots, we have been using reserve supplies."
He added "the situation is tense" but attributed it to panic buying.
"Demand is so high that we aren't managing to keep up," he said.
He also acknowledged there are about three months of reserves that could be used if needed.
With Euro 2016 just a few weeks away, CGT has called for strikes on the national railways and on the Paris subway to be held a week before the football tournament.
Asked on France Inter radio whether further industrial action could cause blackouts in the run up to the championship, CGT leader Philippe Martinez laid the responsibility at the government's door.
He said: "As long as the government refuses to talk, there is a risk that the movement intensifies".
Car owners are already feeling the impact of the strikes as they queue at petrol stations, despite many pumps running dry.
Unions are not only targeting the nation's petrol tanks, but also railways and the electricity network.
Train drivers are staging a one-day strike today, with around one-quarter of high speed trains being cancelled and a similar number of regional and commuter ones.
Workers at a major oil terminal in the port of Le Havre plan to strike on Thursday, as do the employees at the country's nuclear plants, the source of the majority of France's electricity.
State-run Electricite de France would not comment on how supplies around the country will be impacted.
CGT representative Franck Barbay said: "The aim isn't to go on for weeks and weeks".
"Friday there will be general assemblies across the oil sector, from then we'll take the necessary steps, as we always have done, and if we have to carry on we will".
"We're not pleased to lose money but if it's to see this law overturned then we'll have done what was right".
President Francois Hollande's government refuses to abandon a new Labour reform bill and ministers say they will continue to deploy police to clear protesters and ensure petrol supplies.