Allina Health and the union representing 4,800 striking Twin Cities nurses will sit down for talks Tuesday after a federal mediator called on both sides to do so, the two sides announced Friday night.

By then, the nurses will have been on strike for more than three weeks. Their open-ended strike, which began Sept. 5, followed a seven-day strike in June. Talks between the two sides, which have stalled over health insurance issues, began in February.

The exact time and place of Tuesday's talks are as yet undetermined, the Minnesota Nurses Association said in a news release.

The dispute initially centered on Allina's demand to move the nurses from four costly union-backed health plans to its corporate health insurance. Eventually, negotiators for the nurses agreed to drop the union plans but asked for some control of the policies and assurances that the benefit levels of the corporate plans wouldn't erode. Allina rejected that idea, disputing the union's contention that its corporate plans are substandard.

The five affected hospitals are Abbott Northwestern in Minneapolis, United Hospital in St. Paul, Mercy Hospital in Coon Rapids, Unity Hospital in Fridley and the Phillips Eye Institute in Minneapolis.

The nurses have until Oct. 1 to return to work or they will have to pay COBRA rates for their insurance, according to a news release from Allina Health.

In the release, the company thanked 559 nurses who it said have crossed picket lines, as well as temporary nurses and other medical professionals who have continued to work during the strike. The union has questioned Allina's figures on the number of nurses who have crossed picket lines, saying its own count after the June strike was much lower than what Allina reported.

The union has a hardship fund of more than $4 million and has begun processing requests from nurses struggling with the loss of income during the strike.

Staff writer Jeremy Olson contributed to this report.