LOS ANGELES (Reuters) - Shipping lines are planning a partial four-day shutdown of U.S. West Coast ports starting on Thursday, as labor talks between the companies and the union representing 20,000 dockworkers stalled, the companies said on Wednesday.

The affected ports handle nearly half of all U.S. maritime trade and more than 70 per cent of Asian imports.

The impact of port slowdowns has rippled through the U.S. commercial supply chain, disrupting shipments of a wide range of goods affecting agriculture, manufacturing, transportation and retail.

Loading and unloading of cargo vessels at the 29 ports will be suspended again, as they were last weekend, on Thursday and again on Saturday, Sunday and Monday, the companies' bargaining agent, the Pacific Maritime Association, said in a statement.

The PMA said, however, that work would continue at management's discretion in the terminal yards, rail hubs and terminal gates during those days for the clearing of cargo containers stacked up at the harbors.

In addition to suspending vessel operations on Saturday and Sunday, as they did the previous week, the shipping companies plan to extend the partial shutdown to this Thursday and next Monday, which coincide with the Lincoln and Washington presidents day birthday holidays and command premium pay, the PMA said.

The companies, citing chronic slowdowns in freight traffic they blamed on the union, said it made no sense to pay union workers higher holiday and weekend wages "for severely diminished productivity while the backlog of cargo at West Coast ports grows." The companies have accused the International Longshore and Warehouse Union of instigating work slowdowns since October to gain leverage in labor talks that have dragged on for nine months.

The union has denied this and has faulted the carriers themselves for the worsening cargo congestion, pointing to changes in shipping and management practices cited by port authorities as contributing factors.

Union officials, insisting the two sides are close to reaching a settlement, have characterized shippers' weekend cutbacks in vessel operations as public posturing aimed at exaggerating the magnitude of the crisis.

The freight backups have been most pronounced at the twin ports of Los Angeles and Long Beach, the nation's two busiest cargo hubs, accounting for about 43 per cent of all U.S. container traffic.

The chief executive officer of the Long Beach port, Jon Slangerup, said on Wednesday he understood that the two sides could clinch a deal in "a matter of days," and that the talks were stuck on one last point of contention - the issue binding arbitration for settling contract disputes.

"They are working through it," he told Reuters in an interview in New York. "It could be today. They could make an announcement tonight, saying, 'we're there.'"

The companies say union negotiators are seeking the right to dismiss any arbitrator who rules against the union at the end of each contract period, a demand management says is aimed at removing arbitrators who have found the union guilty of more than 200 slowdowns or work stoppages.

A union spokesman has characterized that assertion as "totally inaccurate."