NORTH BAY -- The National Union of Healthcare Workers has issued official notice to Kaiser Permanente that its 4,000 workers across the state intend to hold strikes next week starting Sept. 21 because  of disagreements over ongoing contract negotiations.

In Northern California, approximately 1,500 mental health professionals and optical workers will walk off the job on Sept. 22.

Workers at all of the North Bay Kaiser facilities will be striking, but consolidated picket lines will be held in Santa Rosa, San Rafael and Vallejo, NUHW spokesman Leighton Woodhouse said. The strike would include about 220 workers across the North Bay, at facilities in Marin, Sonoma, Napa and Solano counties.

The California Nurses Association, with some 17,000 registered nurses at Kaiser facilities, will join the union as part of a sympathy strike, according to NUHW, which will amount to the largest strike in Kaiser's history. Workers will walk off the job for one-, two- and three-day durations from September 21 to 23.

In a statement, Kaiser said it was "disappointed" about the union's notice to strike.

"We are disappointed to receive this strike notice from NUHW while much of our negotiations are still in the preliminary stage," said Gay Westfall, senior vice president of human resources for Kaiser. "While we recognize NUHW’s legal right to conduct a strike, we believe the bargaining table is the best place to resolve differences. We are bargaining in good faith and will continue to meet with NUHW over the next few weeks, with the goal to reach an agreement soon."

Mr. Woodhouse disputed the notion that negotiations were preliminary, saying NUHW and Kaiser have been in negotiations since March 2010 in Southern California and since January 2011 in Northern California.