About 4,800 nurses at five Minneapolis-area hospitals began a weeklong strike Sunday over a contract impasse.

Members of the Minnesota Nurses Association began striking at 7 a.m. at the hospitals, all operated by Allina Health. The main dispute is over Allina's effort to switch union nurses to the same health insurance plans as more than 30,000 other Allina employees that carry lower monthly premiums but higher out-of-pocket costs.

No new contract talks are scheduled. The nurses' current contract expired May 31.

The union's president, Mary Turner, said Sunday that members would demonstrate for 12 hours a day outside of each of the five hospitals: Abbott Northwestern in Minneapolis, Mercy in Coon Rapids, United in St. Paul, Unity in Fridley, and the Phillips Eye Institute in Minneapolis.

Allina plans to keep the facilities operating with replacement nurses.

"All Allina Health hospitals are open and caring for patients during the strike," Allina spokesman David Kanihan said in a statement Sunday.

Kanihan said the initial transition to replacement nurses has gone smoothly, and the hospitals were functioning as planned with patient volumes consistent with normal levels for a Sunday in June.

Nurses rejected Allina's latest contract offer and authorized a strike earlier this month, contending the proposal would shift too much of the company's health care costs to the nurses. The company says the move would encourage more frugal use of health care, cutting its costs by $10 million a year.

Other unresolved issues include nurse-to-patient staffing ratios and workplace safety. On pay, Allina is offering nurses 2 percent wage increases in each of three years. The union has proposed 3 percent raises in each of those years.

The last major Twin Cities nurses strike was in 2010, when 12,000 nurses walked out at 14 metropolitan-area hospitals for a day. Nurses at the other Twin Cities hospital systems reached contract agreements months ago.