SAN FRANCISCO -- A frail labor peace between shipping lines and West Coast longshoreman collapsed Sunday when workers were ordered off their jobs indefinitely.

The Pacific Maritime Association, which represents the shipping lines, said it would bar workers from the docks until the union agrees to extend a lapsed contract while talks toward a new agreement continue.

West Coast ports handled more than $300 billion in cargo over the past year. Continued labor unrest could cost the U.S. economy an estimated $1 billion a day.

"They're just doing whatever they're doing," said Steve Stallone, a spokesman for the International Longshore and Warehouse Union.

Association president Joseph Miniace called Sunday's action a "defensive shutdown" that came less than 12 hours after workers returned to the docks.

Shipping lines imposed a lockout Friday immediately after talks broke down. That lockout was lifted, then reimposed after the association accused the union of understaffing operations and dispatching unskilled workers.