Royal Dutch Shell Plc and the United Steelworkers union (USW) negotiated for hours on Tuesday over a new labor contract for U.S. refinery workers, the company said on day 10 of the biggest refinery walkout since 1980.

They have been unable to agree since this year's talks started on Jan. 21 on the size of wage increases and how to monitor worker fatigue, which is tied to accidents and can be handled through overtime pay or adding workers.

Shell is the lead representative for oil companies at the negotiations for a three-year pact that would cover 30,000 workers at 63 plants.

The union wants wage increases, a tighter safety policy, and reductions in non-union contractors working in refineries.

Over the weekend, walkouts widened to include BP Plc's Whiting, Indiana, refinery and its joint-venture refinery with Husky Energy in Toledo, Ohio.

In total, strikes have been called at 11 plants, including nine refineries that account for 13 percent of U.S. refining capacity. About 5,400 workers are now on the picket lines.

Companies have called on trained managers to keep their plants running at nearly normal levels, except for Tesoro Corp , which opted to shut its Martinez, California, refinery as part of an already ongoing overhaul.

Reporting By Erwin Seba and Terry Wade