The White House urged employers and unions to forge a deal "as quickly as possible" to end a nearly week-old strike blocking a key US trade hub amid a warning it was costing billions.
Los Angeles Mayor Antonio Villaraigosa has called for round-the-clock bargaining to end the walkouts at the ports of Los Angeles and nearby Long Beach that began last Tuesday.
Employer and labor union representatives at the ports, which handle more than 40 percent of ocean-shipped US imports from Asia, have met a number of times for negotiations since the weekend, but no progress has been reported.
"This cannot continue," Villaraigosa said in a message to union leader John Fageaux Jr. and employers' chief negotiator Stephen Berry, warning that the labor dispute is "costing our local economy billions of dollars."
"The cost is too great to continue down this failed path," he wrote Sunday.
In Washington, President Barack Obama's spokesman Jay Carney was asked about the strike at his daily briefing.
"We -- and that includes the president -- continue to monitor the situation in Los Angeles closely and urge the parties to continue their work at the negotiating table to get a deal done as quickly as possible," he said.
Pressed on whether Obama could intervene in the dispute, he added: "He is concerned... and we at the White House and broadly in the administration are concerned."
The strike by some 800 clerical staff from the International Longshore and Warehouse Union started at a terminal in Los Angeles port last Tuesday but spread to six other terminals and Long Beach as of Wednesday.
Some 10,000 ILWU members have honored the picket call, shutting down 10 of the 14 cargo container terminals at the complex.
The two ports average $1 billion of cargo each day, a huge proportion of it shipments to and from Pacific nations.
The strike has forced container at least 11 ships so far to divert to other ports in California, notably Oakland up the coast near San Francisco, and Mexico.
The striking workers claim that the Harbor Employers Association wants to outsource jobs, but employers' spokesman Berry said the strike was over "demands that we hire people they don't need."
The National Retail Federation (NRF) noted that the recovery from a 10-day lockout of West Coast ports in 2002 took six months and cost the economy an estimated $1 billion a day.
"An extended strike... could have a greater impact considering the fragile state of the US economy," it said last week, calling for Obama to intervene to resolve the dispute.
"The two sides must remain at the negotiating table until a deal is reached," it added.
The port in Los Angeles and the one in neighboring Long Beach constitute the seventh busiest commercial harbor in the world.