Alternative fuels remain costly for military services.

* Alternative fuels remain expensive as the military services move to diversify their energy portfolios, the Government Accountability Office found.

In a July report tided, "Defense Energy: Observations on DoD's Investments in Alternative Fuels," the watchdog noted that the Defense Department paid $50.50 per gallon last year for alternative jet and naval distillate fuels. For conventional fuels of the same type, the Pentagon paid only $3.91 per gallon.

"DoD is currently purchasing alternative fuels ... at a premium price," GAO said in its report.

It noted that the quantity of conventional fuels purchased by the Defense Department still dwarfs that of alternatives. In 2014, the U.S. military bought 3.3 billion gallons of conventional jet and naval distillate fuels but only 72,000 gallons of alternatives-at a total cost of $12.9 billion and $3.6 million, respectively.

But the Pentagon has ambitious goals for expanding its use of alternative fuels in the coming years. Defense Department officials have argued that such diversification measures are needed to promote energy security.

The Navy has set a goal of deriving 50 percent of total energy consumption from alternative sources, including non-fossil fuels, by 2020. The service estimates that doing so would require using about 336 million gallons of alternative naval distillate and jet fuels on an annual basis by 2020, GAO said.

The Air Force...