The U.S. labor secretary joined congressional leaders, three governors and a big-city mayor on Wednesday in pushing shipping lines and the dockworkers' union to settle a contract dispute that has led to months of turmoil and cargo backups at 29 West Coast ports.

Labor Secretary Tom 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 could, by some estimates, cost the U.S. economy billions of dollars.

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 country's imports from Asia.

More recently, the shipping companies have sharply curtailed operations at the terminals, suspending the loading and unloading of cargo vessels for night shifts, holidays and weekends at the five busiest ports.

Works has continued around the clock in the dockyards, rail yards and terminal gates for most of the ports. 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.

ARBITRATION CITED IN SNAGGED TALKS

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, representing 20,000 dockworkers, has insisted that an accord was near in the negotiations, which a federal mediator was assigned to oversee last month.

Perez joined the talks for the first time on Tuesday, according to his spokeswoman, Xochitl Hinojosa, urging the parties to "come to an immediate agreement to prevent further damage to our economy."

Sources familiar with the situation said Perez huddled with each party separately, then briefly together on Tuesday, and met with both sides again on Wednesday as negotiations and "sidebars" stretched into the evening.

Los Angeles Mayor Eric Garcetti, whose city encompasses the nation's busiest cargo port and lies adjacent to the No. 2 cargo hub at Long Beach, also met with representatives of both sides in San Francisco on Wednesday, his spokesman said.

Meanwhile, the governors of the three West Coast states - California, Oregon and Washington - all Democrats, issued a statement on Wednesday welcoming Perez' involvement and calling for a quick resolution to the dispute.

Separately, eight congressional Republicans who chair House or Senate panels with jurisdiction over transportation and labor sent a letter to Obama on Wednesday urging him to take further unspecified action if a settlement is not reached by March 2 - two months from the date the federal mediator was appointed.

A Senate Commerce Committee spokeswoman, Lauren Hammond, said the letter's reference to "exercising additional leadership to resolve the situation" could be interpreted to mean invoking the 1947 Taft-Hartley Act.

Under that law, a president can seek a federal court order compelling the end to a work stoppage in a labor dispute if it poses a national emergency. But labor law experts have said it would be difficult to make such a case to a judge under current circumstances.

The union and the PMA have declined public comment since a federal mediator called for a news blackout last Friday.

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 under Taft-Hartley.