U.S. Labor Secretary Tom Perez planned to hold a second round of talks with shipping company executives and union leaders for 20,000 dockworkers on Wednesday, seeking to broker a deal to end months of labor turmoil clogging cargo traffic at 29 West Coast ports.

Perez was sent to San Francisco as an emissary of President Barack Obama, who has come under mounting pressure to intervene in a conflict that has reverberated through the trans-Pacific commercial supply chain and, by some estimates, could cost the U.S. economy billions of dollars.

Steadily worsening cargo congestion that the union and shippers blame on each other has slowed freight traffic since October at the ports, which handle nearly half of all U.S. maritime trade and more than 70 percent of the nation's imports from Asia.

More recently, the shipping companies have sharply curtailed operations at the terminals, limiting the loading and unloading of cargo vessels to daytime shifts at the five busiest ports and to non-holiday weekdays only throughout the system.

Daytime work has continued in the dockyards, rail yards and terminal gates. Some smaller ports remained open to nighttime vessel operations as well.

The union and shipping companies each accuse the other side of instigating the disruptions to gain leverage in contract negotiations that have dragged on for nine months, appearing to hit a roadblock in the last two weeks.

The bargaining agent for the shippers and terminal operators, the Pacific Maritime Association, has said talks hit a snag over a union demand for changes in the system of binding arbitration of contract disputes.

The International Longshore and Warehouse Union has insisted the two sides are near an accord.

Perez joined the talks for the first time on Tuesday, meeting separately with each party, then briefly with both sides together, sources familiar with the situation told Reuters.

"Secretary Perez made clear that the dispute has led to a very negative impact on the U.S. economy, and further delay risks tens of thousands of jobs and will cost American businesses hundreds of millions of dollars," Labor Department spokeswoman Xochitl Hinojosa said in a statement at day's end.

Perez urged the parties "to come to an immediate agreement to prevent further damage to our economy," she said.

More talks were scheduled for Wednesday.

The union and the PMA have declined public comment since agreeing last Friday to honor a news blackout requested by a federal mediator who joined the talks last month.

Effects of the port slowdowns have rippled through the U.S. economy, extending to agriculture, manufacturing, retail and transportation.

The last time contract talks led to a full shutdown of the West Coast ports was in 2002, when the companies imposed a lockout that was lifted 10 days later under a court order sought by President George W. Bush.