LOS ANGELES, Jan. 26 (Xinhua) -- Tens of thousands of Kaiser Permanente healthcare workers walked off the job early Monday in the U.S. state of California, launching an open-ended strike that marks the latest labor disruption at the nation's largest not-for-profit integrated healthcare system.

About 31,000 employees, represented by the United Nurses Associations of California/Union of Health Care Professionals (UNAC/UHCP), joined the union walkout. The group includes registered nurses, nurse anesthetists, pharmacists, midwives, physician assistants, therapists, dietitians and other specialized healthcare professionals.

Picket lines began on early morning at dozens of Kaiser facilities across California, such as the northern area's major Medical Centers in Roseville, Santa Clara, and Oakland, where Kaiser Permanente is headquartered.

Additional actions are also planned across Northern and Southern California and at one hospital in Hawaii.

"It was inevitable," Sarah H, a nurse on the West Coast told Xinhua. "We're always understaffed, and the extra work and longer hours lead to burnout."

The strike follows months of stalled contract negotiations over differences and complicated by mutual accusations. Previously, a five-day walkout in October involved more than 40,000 Kaiser workers across several western U.S. states. Union contracts expired on Sept. 30, and talks have continued for more than seven months.

UNAC/UHCP leaders said the latest decision to strike was driven by unresolved concerns over wages, chronic understaffing, workplace safety and burnout.

The union is seeking wage increases totaling 25 percent over four years, while Kaiser Permanente has countered with a proposed 21.5 percent increase, which it has described as its strongest national offer to date.

Kaiser said it has contingency plans in place to maintain patient services during the strike, including using physicians, managers, reassigned staff and temporary clinicians. Hospitals and nearly all medical offices are expected to remain open. Emergency departments will continue operating.

The union estimated that roughly one in four Californians receives healthcare from Kaiser Permanente. ■