WASHINGTON: More than 75,000 employees at Kaiser Permanente began one of the largest healthcare workers strikes in recent US history on Wednesday after failing to resolve a dispute over staffing levels.

The strike of union members at the largest non-profit healthcare organization in the United States began on Wednesday morning at sites in Virginia and the District of Columbia.

It is due to spread to the West Coast later in the day, where the vast majority of the company's workforce is based.

The walkout will be "the largest healthcare worker strike in U.S. history," the Coalition of Kaiser Permanente Unions, an umbrella group representing local unions, said in a statement last month.

The coalition threatened to engage in further strike action in November "if Kaiser continues to commit unfair labour practices."

The union wants pay increases across the board, and protections against subcontracting and outsourcing of labour, among other demands, according to recent statements from the coalition.

In a statement, Kaiser Permanente said it planned to keep its medical centres running throughout the three-day strike.

"We are disappointed that some unions have called on employees to participate in labour strikes," the company announced in a statement posted on its website.

The firm said it had put contingency plans in place but warned customers to expect "longer than usual" wait times.

"Our medical centres will remain open during the strike and will be staffed by our physicians and trained and experienced managers and staff," it added.