The Hartford Financial Services Group Inc. says it has agreed to sell its medical stop-loss insurance business to National Benefit Resources Inc.

NBR is a subsidiary of UnitedHealth Group Inc., Minnetonka, Minn.

The transaction, the terms of which were not disclosed, is expected to close in early April.

The sale will allow The Hartford’s group benefits division to focus on expanding its share of the group disability and life insurance markets, said Lizabeth Zlatkus, president of The Hartford’s international wealth management and group benefits division.

“We have found a great match for our business in NBR, a company that is committed to this marketplace,” she said.

NBR will offer positions to all 102 of The Hartford’s stop-loss employers, according to Zlatkus.

Stop-loss insurance provides protection to self-insured employers against high claim costs.

The Hartford, which has been in the stop-loss business since 1974, has about $200 million of stop-loss premium with over 800 self-funded employer plans.