(Reuters) - Labor Secretary Tom Perez will travel to California to help broker an agreement between shipping companies and dockworkers in a dispute that has led to a partial shutdown of ports along the U.S. West Coast, the White House said on Saturday.

The move by the Obama administration came after shippers vowed to prevent the loading and unloading of freight through Monday from container ships at the 29 ports, barring a settlement in talks with the dockworkers' union.

The impact of the dispute has rippled through the U.S. commercial supply chain, slowing deliveries of a wide range of goods, from agricultural produce to housewares and apparel.

"The negotiations over the functioning of the West Coast ports have been taking place for months with the administration urging the parties to resolve their differences," White House spokesman Eric Schultz said.

"Out of concern for the economic consequences of further delay, the president has directed his Secretary of Labor Tom Perez to travel to California to meet with the parties to urge them to resolve their dispute quickly at the bargaining table."

Perez has been in contact with the parties and will keep the president updated, he said.

On Friday, negotiators for the union representing 20,000 dockworkers at the ports and management's bargaining agent, the Pacific Maritime Association, agreed to a federal mediator's request for a 48-hour news blackout. The two sides held a bargaining session on Thursday that marked their first face-to-face meeting in nearly a week.

The association has said the talks hit a new snag over a demand by the International Longshore and Warehouse Union for changes in the system of binding arbitration in contract disputes.

By Saturday morning, 32 freighters were waiting to dock at the ports of Los Angeles and Long Beach, California, said Lee Peterson, a spokesman for the port of Long Beach.

Shippers first suspended vessel operations at the ports for two days last weekend and again on Thursday, a union holiday. Port operations resumed in full for one eight-hour shift on Friday before the loading and unloading of container ships was halted again.

The West Coast ports were not left entirely dormant. The companies said work continues in the dockyards, rail yards and terminal gates as they seek to clear some of the cargo containers already stacked up on the waterfronts.

(Reporting by Jeff Mason in San Francisco and Alex Dobuzinskis in Los Angeles; Editing by Frances Kerry and Clelia Oziel)