After seven months of unresolved negotiations, West Coast port employers have asked for a federal mediator to step in and guide them through contract talks with the union representing about 20,000 dockworkers at 29 ports, including the ports of Los Angeles and Long Beach.

Unable to reach an agreement with the International Longshore and Warehouse Union, the Pacific Maritime Association requested federal help Monday.

“After seven months of negotiations, we remain far apart on many issues,” PMA spokesman Wade Gates said in a statement. “At the same time, the union continues its slowdowns, walk-offs and other actions that are having impacts on shippers, truck drivers and other local workers — with no end in sight.

“It is clear that the parties need outside assistance to bridge the substantial gap between us.”

ILWU spokesman Craig Merrilees said the union plans to respond today.

Both sides have been negotiating since May over a new contract. Their last contract expired in July.

At the start, they agreed to keep details about the negotiations from the media and vowed to not disrupt day-to-day operations.

The two sides made some progress. In August, they reached a tentative deal on health care benefits, a critical talking point.

But negotiations grew ugly in November when PMA accused the ILWU of work slowdowns at ports in Los Angeles, Long Beach, Oakland, Seattle and Tacoma, saying that dockworkers are working at half-speed at some ports and not dispatching qualified crane operators who could place containers on trucks and trains.

The ILWU has denied PMA’s slowdown allegations.

Meanwhile, the arrival of bigger ships carrying more goods, the lack of available chassis and other issues have contributed to congestion throughout the supply chain.

Container ships remain at sea, waiting to be unloaded. Trucks form long lines at terminal gates for hours, their drivers hoping to pick up cargo, but they are not always successful. Retailers have had todivert cargo from West Coast ports to bypass bottlenecks.

The PMA hoped to resolve talks without needing a federal mediator but said the slowdowns have prompted officials to seek outside help.

Several politicians, industry associations and port leaders have urged President Barack Obama to intervene by exercising the use of a federal mediator.

National Retail Federation Vice President of Supply Chain Jonathan Gold said Monday the nation’s supply chain community welcomes the news and urged the White House to accept the PMA’s call for federal mediation.

“For the last several months, the nation’s retailers, manufacturers, farmers and others have witnessed a dramatic increase in port congestion,” he said in a statement. “The congestion may have many causes, but we believe it is due, in part, to the lack of a new labor contract. … A federal mediator was key to resolving the East and Gulf Coast ports contract in 2012, and it can do the same for the West Coast ports in 2014.”