A U.S. federal mediator says an agreement has been reached in a port dispute, averting for now a strike by thousands of longshoremen at major U.S. ports.

The director of the Federal Mediation and Conciliation Service, George Cohen, says a container royalty payment issue has been agreed upon in principle between the United States Maritime Alliance and the International Longshoremen's Association, subject to achieving an overall collective bargaining agreement.

The FMCS director says significant issues remain but he is “cautiously optimistic” they can be resolved in a 30-day extension period agreed upon by the parties, which ends midnight, January 28.

Longshoremen threatened to strike starting Sunday, which would have disrupted traffic from U.S. ports in the Gulf of Mexico and on the East Coast.

The skilled longshoremen operate high-tech equipment that moves cargo containers swiftly between ships and trains or trucks. Managers say these workers are well paid, and should no longer receive additional payments based on the weight of cargo they handle. These payments started many years ago as a way to compensate longshoremen for the jobs and hours that were lost when ports became highly automated.