A federal mediator is calling negotiators for Allina Health and roughly 4,800 hospital nurses back for a last-chance round of talks on Friday.

Absent a deal, the nurses are scheduled to go on strike at 7 a.m. Monday — Labor Day — at United Hospital in St. Paul, Mercy Hospital in Coon Rapids, Unity Hospital in Fridley, and Abbott Northwestern Hospital and the Phillips Eye Institute in Minneapolis.

The nurses staged a one-week strike in late June, but the latest walkout is considered open-ended.

Allina leaders are recruiting temporary nurses through large national staffing agencies in order to maintain key medical services in the hospitals during any walkout. The June strike cost Allina more than $20 million.

Health insurance has been the key stumbling block in the failed talks to date, with Allina wanting to end four union-backed health plans and move nurses over to its corporate benefit plans — at an estimated savings of $10 million per year. Nurses have balked at the proposal — some because they prefer their current benefits, others because they want Allina to make other concessions first.