The strike, expected to last three days and described by the Minnesota Nurses’ Association as one of the largest in United States history, highlights nationwide health worker shortages that have been exacerbated by the Covid-19 pandemic.

The union says it has been negotiating a new agreement for more than five months and nurses have been working without a contract for weeks.

“Hospital executives have already driven nurses away from the bedside by their refusal to solve the crises of staffing and retention in our hospitals,” the union’s negotiating team said in a statement, adding that nurses were “understaffed and overworked”.

The walkout was expected to affect at least 13 hospitals around Minneapolis and neighbouring St Paul. Twin Cities Hospital Group, which oversees four hospitals where nurses are on strike, said it had asked the nurses’ union to join them in mediation.

“A trained mediator can help parties focus on the key elements needed to move forward together. However, the nurses’ union has rejected all our requests for mediation,” the group said on its website.

Allina Health, which owns four hospitals involving nurses on strike, said on its website it was making every effort to minimise disruptions to patient care. It said it had been expecting the strike for months.

The nurses’ strike has drawn support from US legislators in Washington, including veteran Senator Bernie Sanders. “Nurses are the backbone of our healthcare system,” Sanders wrote on Twitter, calling for fair scheduling and higher wages.

According to the US Bureau of Labor Statistics (BLS), employment in healthcare across the United States is still below pre-pandemic levels. At least 37,000 fewer people work in healthcare now than in February 2020, according to a September 2 report from BLS.

(Reporting by Rami Ayyub; Editing by Aurora Ellis.)