Strike action disrupted work at two key West Coast ports, threatening a key gateway for trade to and from Asia, union leaders and harbor officials said.
The action by clerical staff started on Wednesday at a terminal in the port of Los Angeles but spread to six other terminals and the nearby port of Long Beach during the day.
The two ports deal in $1 billion of cargo per day on average, a huge proportion of it shipments to and from Pacific nations.
Workers, who initially walked off the job on Tuesday over staffing and contract issues, refused an arbitrator's ruling that they should return to their desks on Wednesday.
Unions led by the International Longshore and Warehouse Union expanded their picket action to 10 of 14 terminals at the two ports. Other staff, including crane operators, refused to cross picket lines.
A Port of Long Beach spokesman, Art Wong, confirmed that the strike had halted the movement of cargo at three Long Beach terminals.
The striking workers claim that the Harbor Employers Association wants to outsource jobs, but employers' spokesman Stephen Berry said the strike was over "demands that we hire people they don't need."
He added that clerks get 11 weeks holiday per year and have an absenteeism rate of 29 percent.
Union spokesman John Fageaux said: "We're hoping to get our people back to work... but can't let these big corporations outsouce our jobs."
The port in Los Angeles and the one in neighboring Long Beach constitute the seventh busiest commercial harbor in the world, handling more than 40 percent of ocean-shipped US imports from Asia, according to the LA Times.