Strikers have blockaded France's Mediterranean energy hub and closed oil major Total's biggest refinery in the country, a major fuel supplier to Europe and the United States.

In the United States, a dozen refineries accounting for 18 percent of capacity remain shut after Hurricanes Rita and Katrina. This equals a daily loss of more than 1.3 million barrels of gasoline.

U.S. crude oil was up 34 cents to $66.58 a barrel by 0540 EDT. London Brent crude gained 35 cents to $63.83 a barrel.

Washington said last week up to 15 percent of U.S. refining capacity could be out for at least another couple of weeks.

"Both in terms of gasoline availability and prices of natural gas and heating oil, we're going to have some problems," U.S. Energy Secretary Samuel Bodman said in an interview published on Monday.

The latest U.S. government figures showed gasoline stocks were down 5.9 million barrels from a year ago. Heating fuel inventories remained higher than a year earlier, but were down from the previous week, at a time when companies usually start building stocks to meet peak winter demand.

Crude and gas output from the Gulf of Mexico, home to more than a quarter of U.S. domestic output, may also be slow to return. As of Friday, 98 percent of crude production and 79 percent of natural gas output was shut in the Gulf of Mexico.