Amid gripes from frequent fliers about difficulty using their miles for free flights, airlines are making some changes and adding features in an effort to keep travelers loyal.

In a ranking of major U.S. airline frequent flier programs released by U.S. News and World Report, Delta's SkyMiles program ranked No. 2 for the second year in a row.

U.S. News  evaluated the programs based on aspects like the size of their networks and the speed at which users can get enough points or miles to earn a free flight or hotel.

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Delta came in behind No. 1 ranked Alaska Airlines, and ahead of JetBlue, American and Southwest frequent flier programs in the U.S. News ranking.

Southwest's Rapid Rewards program has ranked higher in some other loyalty program rankings, based on the ease of finding reward seats.

As some frequent fliers face frustration with redeeming miles for flights, Delta has added other uses for miles. Earlier this year, Delta began allowing frequent fliers to redeem their miles for Delta gift cards. But those who get the most benefits in Delta's frequent flier program are elite medallion members who reach a certain threshold of spending and flying.

The U.S. News Best Airline Rewards Programs are below. More information is at USNews.com.

  1. Alaska Airlines' Alaska Mileage Plan program
  2. Delta Airlines Delta SkyMiles program
  3. JetBlue Airline's TrueBlue program
  4. American Airlines' AAdvantage program
  5. Southwest Airlines' Rapid Rewards program
  6. United Airlines' MileagePlus program
  7. Frontier Airlines FRONTIER Miles program
  8. Hawaiian Airlines' HawaiianMiles program
  9. Spirit Airlines' Free Spirit program