Thousands of nurses launched on Monday one of the largest strikes in the United States history, complaining of bad conditions, low salaries and understaffing worsened by the strains of the coronavirus pandemic.

Some 15,000 nurses at seven health care systems in the Minneapolis and Duluth areas walked out. The affected hospitals said they recruited temporary nurses and expected to maintain most services.

The strike, slated to last three days and described by the Minnesota Nurses Association as one of the largest in U.S. 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 that 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."

Picket signs went up and strike chants began at 7 a.m. outside 15 Twin Cities and Duluth area hospitals. The hospitals and the striking nurses said staff shortages are a shared concern.

Minnesota Nurses Association President Mary Turner said pay raises are necessary to address the "crisis of retention" that would otherwise leave the hospitals severely understaffed.

The hospitals have offered 10%-12% wage increases but the nurses are seeking more than 30%. Hospital leaders called their wage demands unaffordable, noting that Allina and Fairview hospitals have posted operating losses and that the cost of such sharp wage increases would be passed along to patients.

"The union rejected all requests for mediation and held fast to wage demands that were unrealistic, unreasonable and unaffordable," several of the Twin Cities hospitals under strike said in a joint statement.

Second-year nurse Madi Gay, who was picketing Monday morning after completing her overnight shift, that she had already reduced her hours at M Health Fairview Southdale Hospital because of the stress and frustration of caring for so many severely ill patients.

"How long can you keep this up?" Gay asked. "My license is on the line."

Union spokesperson Sam Fettig said the nurses settled on a three-day rather than open-ended strike because of concerns about the impact of drawn-out labor action on patient care.

"As the nurses always say, this isn’t something they want to be doing, they want to be at the bedside providing care," Fettig said.

Hospitals argued that the proposals by the union and its nurses are too costly.

"It just isn’t a realistic number," Paul Omodt, a spokesperson for several of the Minneapolis-area hospitals, said last month.

The hospitals affected by the strike included those operated by Allina Health, M Health Fairview, Children’s Hospital, North Memorial and HealthPartners. In Duluth, it was Essentia and St. Luke’s.

As union members in Minnesota walked the picket line, UW Health nurses and the UW Hospital board in Wisconsin approved an agreement intended to avert a strike that had been scheduled to start Tuesday, a spokesperson for the nurses’ union said Monday.

Nurses and administrators didn’t immediately release details of that agreement. The hospital board and 97% of nurses who voted approved the deal, according to SEIU Healthcare Wisconsin.

According to the U.S. Bureau of Labor Statistics (BLS), employment in health care across the United States is still below pre-pandemic levels. Some 37,000 fewer people work in healthcare now than in February 2020, a Sept. 2 report from BLS said.

The Minnesota nurses’ strike comes amid an upsurge in union activity nationwide.

A national railroad strike could begin as early as Friday unless Congress steps in to block it. The two largest railroad unions have been demanding that the major freight carriers go beyond a proposed deal recommended by arbitrators appointed by President Joe Biden.

Some high-profile companies, including Starbucks, are among those trying to stifle. Since late last year, more than 230 U.S. Starbucks stores have voted to unionize. Starbucks opposes unionization.