Grocery retailer Kroger (KR) on Friday threatened to sever ties with Cigna's Express Scripts pharmacy by the end of the year unless a new deal is negotiated.

Kroger called the current agreement an "unsustainable" pricing arrangement.

The Cincinnati, Ohio-based retailer said it wanted to reach an agreement that would keep prices affordable for its 17 million customers—in particular members of the military and their families.

"We do not believe Kroger customers should have to pay higher costs to increase Express Script's profits," Colleen Lindholz, Kroger Health's president said in a statement.

However, Kroger said it "remains willing to negotiate" with Express Scripts. According to the grocery retailer, more than 90% of Kroger's customers will not be affected by the termination of the deal.

However, failure to reach a new agreement by Dec. 31 will mean that Express Scripts' customers will be unable to fill prescriptions within the Kroger network starting next year.