Union-led work slowdowns could shut down the 29 U.S. West Coast ports in five to 10 days, the head of the shippers’ association said, urging the union to accept a new offer that includes 3 percent raises.

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.

Traffic at Canada's Port of Prince Rupert soared 49 percent in July from a year earlier as container ships try to avoid delays from a possible strike by U.S. West Coast longshoremen.

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.

Twelve years after a labor dispute closed West Coast ports and cost the U.S. economy $1 billion a day, negotiators on both sides want to avoid a repeat that could be twice as expensive.