MINNEAPOLIS — Thousands of nurses at five Minnesota hospitals on Monday began an open-ended strike in a dispute over health insurance, workplace safety and staffing.

Clad in red T-shirts, members of the Minnesota Nurses Association, which represents about 4,800 nurses at five Twin Cities-area hospitals run by Allina Health, hit the picket lines at 7 a.m. on Labor Day. They were joined by supporters from other unions. A 22-hour bargaining session ended without agreement early Saturday, and no new talks are scheduled.

Allina says it plans to operate the affected hospitals at normal capacity with around 1,500 temporary nurses.

The affected hospitals are Abbott Northwestern and the Phillips Eye Institute in Minneapolis, United in St. Paul, Unity in Fridley and Mercy in Coon Rapids. The three main trauma centers in the Twin Cities area — Hennepin County Medical Center in Minneapolis, Regions Hospital in St. Paul and North Memorial Medical Center in Robbinsdale — are not affected, nor are several other major hospitals in other networks.

“We have a responsibility to continue delivering high-quality care for the patients and communities who rely on us every minute of every day. And we are fully ready to do so,” Dr. Penny Wheeler. Allina’s president and CEO, said in a statement.

The core issue in the dispute is that Allina wants to transition its nurses by 2020 to the same health plans that cover other employees, in hopes of saving about $10 million a year. Those plans have lower premiums but higher deductibles and other out-of-pocket costs.

But the union’s executive director, Rose Roach, told members in an open letter that Allina still wants to shift too many costs onto nurses without adequately compensating them for the higher costs they would incur. She said their negotiators proposed at the last talks a plan for eventually switching to the corporate plans, while preserving the nurses’ current plans for as long as possible, but that Allina refused.

“I am so proud of the courageous nurses who serve you on your negotiations team — they fought, they yelled, they calculated, they cried,” Roach said. “Our hearts are heavy sisters and brothers, we tried, we really did.”