"ILWU Local 13 withheld labor again for this morning's shift. The action by the union has effectively shut down the Ports of Los Angeles and Long Beach," the Pacific Maritime Association said in a statement.

The terminal closures come as cargo volumes have dropped sharply at the Southern California ports from peak levels a year ago. The threat of delays during contract talks has spurred retailers, manufacturers and other importers to avoid potential shipping snarls by diverting goods to the East and Gulf coasts.

Still, an extended shutdown could disrupt supply chains. The National Retail Federation said it has renewed a call to the White House to engage in the labor negotiations and prevent disruptions to cargo flows in a statement Friday.