Nearly 13,000 healthcare employees at five University of California medical centers plan to strike on Tuesday in a move that threatens to back up emergency rooms and already has forced the postponement of elective surgeries.

Vocational nurses, respiratory therapists and radiology technologists say they will walk off their jobs for two days to draw attention to issues they tried unsuccessfully to address at the negotiating table - "chronic understaffing and reckless cost-cutting," said Todd Stenhouse, spokesman for the American Federation of State, County and Municipal Employees.

Some of the public hospital system's estimated 3,400 pharmacists, social workers, psychologists, occupational therapists and lab scientists also are expected to walk out Tuesday as part of a one-day sympathy strike at the five medical centers in San Francisco, Davis, Los Angeles, San Diego and Irvine.

"Leaders of both unions claim their chief concern is patient care but it's very simple: If they strike, services to patients suffer," said Dwaine Duckett, vice president for human resources for the university.

He said the walkout had forced the postponement of hundreds of surgeries, including five pediatric heart surgeries and procedures like chemotherapy infusions for children with cancer.

Stenhouse said the striking workers, who have been working without a contract since September, will be ready to return to the hospital in the event of an area-wide emergency.

"We're not trying to hurt UC," he told Reuters, saying the workers had never before walked off their jobs. "We're trying to save it.

"Our top concern is about safe staffing, and we need to put a stop to the diversion of resources away from patient care," he said.

Duckett said the problem was the "union's ongoing resistance to UC's pension reforms," which include raising employee pension contributions, revising eligibility rules for retiree health benefits and creating a second tier of pension benefits for new workers.

Duckett said the university system had offered the workers a four-year contract with up to 3.5 percent annual wage increases. The average employee in the striking union earns $55,000 a year, he said.

California's labor board asked a judge to intervene and order the employees to continue to work.

Sacramento Superior Court Judge David I. Brown found on Monday that about 100 burn center respiratory therapists and poison-control pharmacists were essential workers and barred them from striking. But he refused to stop the other workers from their labor action.

Stenhouse said the union has asked the medical centers to form committees to determine enforceable, safe-staffing levels.