(Reuters) - United States Cellular (USM)said on Thursday it has agreed to sell a portion of spectrum licenses used to transmit mobile phone signals and high-speed data services to AT&T (T) in a $1.02 billion deal.

The transaction is part of the regional wireless carrier's strategy to monetize its spectrum assets that were not part of the previously announced sale to T-Mobile.

In May, U.S. Cellular entered into an agreement with T-Mobile to sell almost all of its wireless operations including customers, stores and 30% of its spectrum assets in a deal valued at $4.4 billion.

Last month, U.S. Cellular agreed to sell select spectrum licenses for $1 billion to Verizon. It also signed deals with two other mobile network operators, but did not disclose the details.

The latest agreement "adds a fourth mobile network operator, in addition to T-Mobile, to the list of those whose subscribers will benefit from the sale of our spectrum licenses," U.S. Cellular CEO Laurent Therivel said on Thursday.

Including the proposed T-Mobile transaction, U.S. Cellular has reached agreements to monetize about 70% of its total spectrum holdings, excluding high-frequency mmWave spectrum, the company said in a statement.

(Reporting by Harshita Mary Varghese in Bengaluru; Editing by Shailesh Kuber)