WASHINGTON — Federal mediators seeking to avoid a walkout of thousands of East Coast and Gulf Coast dockworkers from Massachu-setts to Texas have called a meeting of the union and shipping companies.

The Federal Mediation and Conciliation Service said late Monday that Director George Cohen called the meeting of the International Longshore-men's Association and the U.S. Maritime Alliance before the Saturday 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, 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. Other ports that would be affected 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 Federa-tion 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.

A strike would be the first by the ILA in 35 years.

U.S. military shipments and so-called “bulk” cargo that is not carried in 20-foot to 40-foot long steel cargo containers would not be affected. But the vast majority of the goods sold by U.S. retailers would be affected, as would some of the country's agricultural exports.

One economist called the dispute a contest of wills between some of the world's biggest cargo operators and one of the nation's strongest labor unions.

“The shipping industry is trying to take back some of the power,” said economist John Husing, founder of Economics and Politics Inc. in Redlands, Calif., “but they are up against a union that has abnormal power for its size and one that is in a very strong position.”

The impact of a strike would be mitigated by one thing: This is the slowest season for cargo coming by sea into the U.S. Shippers have usually moved their goods for the busy holiday retail season by October.

Even so, a "failure to reach a contract agreement would result in a coast-wide shutdown at 14 containerized ports — from Maine to Texas — which would have serious economy-wide impacts," the retail federation and coalition of national and state organizations said in a letter sent last week to President Barack Obama.

Late last month, most of the Port of Los Angeles and half of the Port of Long Beach were shut down during an eight-day strike by the clerical unit of the International Longshore and Warehouse Union.

The ILWU, which represents West Coast dockworkers, is not affiliated with the ILA.