WASHINGTON, D.C. (AP) — Federal mediators seeking to avoid a walkout of thousands of East Coast and Gulf Coast dockworkers from Massachusetts to Texas have called a meeting of them and shipping companies.

The Federal Mediation and Conciliation Service said Monday that Director George Cohen called the meeting of the International Longshoremen’s Association and the U.S. Maritime Alliance before the Dec. 29 expiration of the dockworkers’ contract extension. It said the parties have agreed to attend the meeting but wouldn’t elaborate.

Talks between the dockworkers and the shipping companies broke down Dec. 18, just weeks after a critical West Coast port complex was crippled by a strike involving a few hundred workers. Issues including wages are unresolved, but the key sticking point is container royalties, which are payments to union workers based on cargo weight.

Port operators and shipping companies, represented by the Marine Alliance, want to cap the royalties at last year’s levels. They say the royalties have morphed into a huge expense unrelated to their original purpose and amount to a bonus averaging $15,500 a year for East Coast workers already earning more than $50 an hour.

The longshoremen’s union says the payments are an important supplemental wage, not a bonus.

The union represents 14,500 workers at more than a dozen ports extending south from Boston and handling 95 percent of all containerized shipments from Maine to Texas, about 110 million tons’ worth.

The New York-New Jersey ports handle the most cargo on the East Coast, valued at $208 billion last year. The other ports that would be affected by a strike are Boston; Delaware River; Baltimore; Hampton Roads, Va.; Wilmington, N.C.; Charleston, S.C.; Savannah, Ga.; Jacksonville, Fla.; Port Everglades, Fla.; Miami; Tampa, Fla.; Mobile, Ala.; New Orleans; and Houston.

Retailers fear another strike could have catastrophic effects and have asked President Barack Obama to intervene.

The National Retail Federation wrote to Obama last week and asked him to use “all means necessary” to head off a strike.

“We foresee this as a national economic emergency, to be honest,” said Jonathan Gold, the group’s vice president of supply chain and customs policy.

Billions of dollars in commerce at businesses nationwide could be affected, from auto manufacturers awaiting parts to the truckers who deliver them, Gold said.

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