Nurses at five Allina Health hospitals will go on strike starting at 7 a.m. on Labor Day, their union said Friday in a mandatory 10-day notice sent to the health system.

The walkout would be the second by the Allina nurses since negotiations started in February over a new three-year contract. Following a one-week strike in June, the nurses rejected a contract offer from Allina and authorized strike planning in voting last week. But negotiators with the Minnesota Nurses Association (MNA) opted to wait to set a date pending the outcome of talks that occurred Tuesday.

Union leaders formally announced the strike at a news conference at the Minnesota State Fair. They were joined by other Twin Cities labor leaders, who warned Allina that they intend to support the walkout.

Rose Roach, MNA executive director, said the talks on Tuesday produced nothing new: Allina wanted steep concessions on health benefits before addressing the nurses’ requests to improve staffing and workplace safety.

“It was more of the same,” she said. “Concede on our issues, then maybe we will talk to you about yours.”

The strike, by as many as 4,800 nurses, would include Abbott Northwestern Hospital and Phillips Eye Institute in Minneapolis, United Hospital in St. Paul, Mercy Hospital in Coon Rapids and Unity Hospital in Fridley.

WHAT’S AT STAKE Timing: It would begin at 7 a.m. on Monday, Sept. 5. Unsettled issues: Health benefits, staffing and workplace safety. Who would strike: Up to 4,800 nurses at hospitals in Minneapolis, St. Paul, Coon Rapids and Fridley.

Later Friday, Allina released a statement expressing disappointment and criticizing the nurses for “walking away from patient care instead of working to find a reasonable solution.”

“The union’s actions bring us no closer to a settlement,” the statement read, “and do a disservice not only to nurses but to all Allina Health employees and, most importantly, to our patients.”

The newly announced strike is open-ended, meaning that it will start Sept. 5 and continue until a deal is reached.

A 10-day notice is required so Allina can hire temporary nurses to maintain patient care. When the nurses conducted a seven-day strike in June, Allina recruited 1,400 replacements from around the country to cover the week. The strike cost the health system more than $20 million, according to an Allina financial report.

Nurse staffing agencies, including HealthSource Global and Huffmaster, indicate on their websites that they are currently recruiting nurses to cover an Allina strike.

Negotiators for Allina and its hospital nurses have been deadlocked over health benefits, with the health system initially demanding that the nurses drop their four union-backed health plans and switch to its corporate plans. The transition would save the health system an expected $10 million per year, Allina says, but nurses have fought to retain the plans, which charge high premiums up front but low costs afterward when nurses and their families get sick and need care.

“They don’t seem to realize that these insurance plans are why nurses continue to work for Allina,” said Angela Becchetti, an Abbott Northwestern rapid-response nurse who is on the union negotiating team. “The insurance is why we continue to be hit, punched, kicked and spit on. It’s why we can take care of our patients without ever thinking of our own safety.”

In earlier talks, the two sides agreed to retain two of the union plans, but couldn’t agree on how much the nurses should pay in future cost increases.

On Tuesday, they also disagreed on whether new Allina nurses could choose the union plans when they are hired. With a federal mediator ping-ponging between the two sides, union leaders asked that new nurses at least be given a one-time shot to choose the union plans, but Allina officials declined.

Both sides acknowledge that the union plans would eventually die out without new enrollees and with continued cost increases. Allina officials dislike the union plans, which lack cost controls such as higher deductibles that motivate efficient purchases — such as equivalent generic drugs over more expensive brand-name versions.

Nurses also have asked for required annual safety training on how to de-escalate agitated and potentially violent patients, a guarantee of a security guard working in emergency rooms 24 hours a day, and a change in staffing so that charge nurses don’t have direct patient assignments and can instead maintain supervisory roles and fill in for nurses when they need breaks or become busy with other patients.

Allina officials say they are open to changes in staffing, such as an electronic acuity system that assesses the severity of patients’ illnesses in recommending the number of nurses needed per shift. The health system also has agreed to more nurse representation on hospital committees that address the rising problem of assaults and violence by patients and families in hospitals.

However, Allina spokesman David Kanihan said it would be unusual to mandate security guard staffing in a labor contract for nurses. Allina hospitals already require eight hours of safety training every two years for nurses in mental health and emergency units, and two hours for others who can complete additional training on a voluntary basis.

“The union has said that safety is something that we have refused to deal with in these negotiations,” he said, “and that is simply not true. There have been proposals and counterproposals back and forth on the topic of workplace safety in these negotiations.”

The strike declaration follows charges filed by the MNA with the National Labor Relations Board, claiming that Allina has conducted “unfair labor practices” such as withholding key financial information from the union. If the labor board rules that the walkout is an “unfair labor practice” strike, that could protect striking nurses from being permanently replaced.

The lack of progress in contract talks also raises the specter that a mediator will declare an impasse in negotiations, a legal finding that would allow Allina to simply impose its latest contract offer on the nurses.

Asked Friday about the possibility, Roach said that the nurses intend to strike for “as long as it takes” and that they will fight any effort to impose a contract.

It is unclear whether an impasse could be declared while the union’s unfair labor practice charges are pending.

“Really, that’s a nuclear option,” Roach said.

Kanihan agreed: “While we could at some point go that route, we’d rather continue trying to work with the union to get to an agreement our nurses support.”