Shipping executives and union leaders for dock workers at 29 U.S. West Coast ports remained locked in a dispute on Friday as talks, brokered by the U.S. labor secretary, headed into a fourth straight day.
 
Negotiations ended late Thursday with no deal, two sources close to the talks told Reuters Friday, adding that there is more work to do in discussions set to resume in the morning.
 
U.S. Labor Secretary Tom Perez joined the talks in San Francisco on Tuesday at the behest of President Barack Obama, who has come under growing pressure to intervene in a nine-month-old dispute that has rippled through the trans-Pacific commercial supply chain and could cost the U.S. economy billions of dollars.
 
The International Longshore and Warehouse Union, representing 20,000 dock workers, has been locked in negotiations with the bargaining agent for shippers and terminal operators, the Pacific Maritime Association.
 
Tensions arising from the talks have played out in worsening cargo congestion that has severely slowed freight traffic at ports that handle nearly half of all U.S. maritime trade and more than 70 percent of imports from Asia.
 
 
Mayor Libby Schaaf of Oakland, whose San Francisco Bay port is one of several bearing the brunt of the disruptions, told Reuters on Thursday Perez told her and other mayors on a conference call he was trying harder to squeeze a deal and that if a deal was not reached within 24 hours, "he was going to force the two sides to come to Washington to negotiate."