(BCN) -- Hundreds of Northern California Kaiser Permanente nurses started a one-day strike Monday morning over contract talks and concerns about staffing levels.

More than 600 nurse midwives and nurse anesthetists from 20 hospitals took to the picket lines at Kaiser facilities in Oakland and Roseville at 7 a.m., according to officials with the United Nurses Associations of California/Union of Health Care Professionals.

The nurses, who are negotiating their first union contract with the health care system, said in a news release that Kaiser has "refused to settle a fair contract that addresses unsafe staffing, burnout, and the risk to patient care."

"We've had operating rooms sit empty because there weren't enough nurse anesthetists to cover cases," said Jeff Cathcart, a Kaiser nurse in

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