The union for 20,000 dockworkers and a group of their employers at 29 U.S. West Coast ports say they are making slow but steady progress in months-long contract talks seen by the shipping industry as a contributing factor in chronic cargo backups.

Bargaining on a new contract for West Coast longshoremen has intensified after dockworkers who handle a majority of the nation’s cargo began selective slowdowns ahead of the holiday season.

Tension over prolonged labor talks between maritime shippers and the union representing 20,000 West Coast dockworkers has erupted in public as the two sides blamed each other for escalating cargo congestion at the two busiest U.S. container ports.

U.S. retailers appealed to President Barack Obama to intervene in contract negotiations between West Coast dockworkers and maritime companies after a work slowdown spread to the nation’s largest container hub ahead of the holiday shopping season.

Pacific Coast dockworkers negotiating a new labor agreement have begun a work slowdown in Seattle and Tacoma, management said, in the first large job action since their contract expired in July.

The union that represents workers along the St. Lawrence Seaway has agreed to take a contract dispute to arbitration, avoiding a strike that would have shut the waterway linking the Great Lakes and the Atlantic Ocean.

Union workers on the St. Lawrence Seaway, the waterway that links the Great Lakes and the Atlantic Ocean, have given notice of a possible strike, which could start on Friday afternoon, Unifor, Canada's largest private sector union, said on Tuesday.

Union workers who load grain at U.S. West Coast terminals will return to work on Wednesday after ratifying a four-year contract that included work rule changes and wage increases, the union said on Tuesday.

Unionized port workers and a group of Pacific Northwest grain companies reached a tentative contract agreement, according to an announcement on Tuesday

Imports into U.S. ports may hit a five-year high this month, as retailers rush in holiday goods to avoid potential disruptions as West Coast longshore workers negotiate a new contract.