(Reuters) - Bernard J. Tyson, chairman and chief executive officer of not-for-profit health insurer Kaiser Permanente, died unexpectedly in his sleep on Sunday, aged 60, the company said in a statement.

Tyson, who held the top job since 2013, was Oakland, California-based Kaiser Permanente's first black chief executive and a strong proponent for affordable and accessible healthcare.

The company did not give a cause of death. A company spokesperson did not immediately respond to a request for comment.

A day earlier on Saturday, the San Francisco native took to Twitter to post about "high-tech and high-touch" healthcare.

Tyson was described by colleagues in a company statement as "an outstanding leader, visionary and champion for high-quality, affordable health care for all Americans."

In response to Tyson's death, the National Union of Healthcare Workers postponed a five-day strike by 4,000 mental health professional that had been set to begin on Monday at more than 100 Kaiser clinics across California.

"Our members dedicate their lives to helping people through tragedy and trauma, and they understood that a strike would not be appropriate during this period of mourning and reflection," union president Sal Rosselli said in a statement.

The union, locked in a contract dispute its leaders say centers on demands for improved retirement and health benefits for Kaiser's mental health clinicians, has yet to reschedule the strike, he said.

Kaiser's annual operating revenue as of June was nearly $80 billion, up from about $53 billion in 2013 when Tyson took over as CEO, according to the company's website.

The company, which serves more than 12 million people across the United States and was founded in 1945, named Gregory Adams, executive vice president, as interim chairman and CEO.

(Reporting by Melissa Fares in New York; Editing by Richard Pullin and Christopher Cushing)