More than 75,000 workers of the United States' largest non-profit health care organization began a three-day strike Wednesday to demand higher pay and better working conditions.

Kaiser Permanente strikers in Los Angeles, California on Wednesday said they were underpaid and overworked.

"Kaiser Permanente has reported billions of dollars in profits, so all we're asking from Kaiser CEOs is to share those profits with the frontline workers," striking nurse Kathy Lozoya said.

"All we're asking is a fair contract so that we can be able to live."

Kaiser Permanente locations in California, Colorado, Oregon and Washington state were expected to be affected by the three-day strike. A small number of workers in Washington, DC and Virginia were set to walk out for 24 hours.

Kaiser said centers would remain open, but warned there would be "longer-than-usual" wait times.

A Kaiser spokesperson told journalists on Tuesday that talks were continuing.

"Several agreements over specific provisions have been reached" with the Coalition of Kaiser Permanente Unions, the spokesperson said, adding that negotiators were prepared to meet around the clock "until we reach a fair and equitable agreement."

The union, which says this will be the largest health care worker strike in US history, is pushing for pay increases and protections against subcontracting and outsourcing of labor.

It has threatened to engage in further strike action in November "if Kaiser continues to commit unfair labor practices."