LOS ANGELES » West Coast seaports that were all but shut over the holiday weekend because of a contract dispute are reopening, as the nation’s top labor official begins his efforts to solve a stalemate between dockworkers and their employers that already has disrupted billions of dollars in U.S. international trade.

U.S. Secretary of Labor Thomas Perez flew Monday to San Francisco and plans to meet Tuesday with negotiators for both the dockworkers’ union and the maritime association, which represents shipping lines that carry cargo and port terminal operators that handle it once the ships dock.

The two sides began meeting in May, and in recent weeks their disagreements at the bargaining table have led to historically debilitating problems moving cargo through 29 seaports from Southern California to Seattle.

After a fruitless meeting Friday between the International Longshore and Warehouse Union and the Pacific Maritime Association, President Barack Obama said Saturday that Perez would come west and engage in the talks, which a federal mediator has overseen since early January. Over the weekend, Perez was in touch by phone with both sides.

The ports are a critical trade link with Asia and the gateway not just for imports such as electronics, household goods and clothing but also U.S. exports including produce and meat.

Starting Saturday, companies locked out workers who would load or unload ships, saying they would not pay weekend or holiday wage premiums to crews they accuse of intentionally slowing work to gain bargaining leverage. As a result, cranes that would otherwise be moving containers onto dockside yards were raised up, stationary and eerily quiet on normally bustling waterfronts.

It was a broadening of a partial lockout that began in January, when the maritime association stopped calling night crews to load and unload ships, saying smaller crews would instead focus on moving onto trucks and trains containers that already had been unloaded onto cargo-choked dockside yards.

The union denies slowing down and says its members want to work. It blames troubles moving cargo on larger problems with the supply chain, including a shortage of truck beds to carry containers to distribution warehouses.

On Monday, massive ships continued lining up outside the ports, laden with imports now delayed by weeks. Off the coast of Los Angeles and Long Beach, for example, 33 "congestion vessels" were awaiting space at the docks, according to the Marine Exchange of Southern California — a new high since this round of disruptions began.

The last contract expired in July. The two sides have reached tentative agreements on many of the key issues, but are stuck on whether to change the process of arbitrating workplace disputes.

Several other issues have been on the table, including pay. The maritime association says average wages exceed $50 an hour; the union says wages are set between $26 to $36 an hour — though many shifts carry a premium over that range.