Washington ? Squeezed by the soaring cost of oil-based jet fuel, the Air Force is converting its gas-guzzling fleet of aircraft to synthetic fuels and encouraging the creation of a liquefied coal industry that could tap the nation’s vast coal reserves.

This could mean a lucrative new market for coal-producing states, such as Wyoming, Kentucky, Montana, Illinois, Pennsylvania, Texas and West Virginia. But advocates of liquefied coal face a counterattack from environmentalists in the debate over global warming and must prove that they can produce an ecologically friendly product with a low carbon footprint.

Air Force officials have been testing synthetic fuels based on coal or natural gas. They plan to certify the fleet of nearly 6,000 aircraft to fly on a 50-50 blend of synthetic fuel and traditional petroleum-based jet fuel by 2011.

Assistant Air Force Secretary Bill Anderson said the search for affordable, cleaner-burning alternative fuels was driven by economic and national security concerns. The Air Force wants to comply with President Bush’s mandate to end America’s dependence on foreign oil while escaping soaring fuel prices.

For the Air Force, which consumes more than half of all the fuel that the U.S. government uses, the cost of fueling fighters and transports is stratospheric. Every $10 increase in the price of a barrel of oil costs the Air Force another $600 million, Anderson said.

Last year, the Air Force spent $5.8 billion to buy 2.6 billion gallons of fuel. In 2003, the service spent about half that – $2.9 billion – to buy slightly more fuel, nearly 3 billion gallons.

The synthetic fuel is developed from a technology known as the Fischer-Tropsch process, which can convert coal, natural gas or biomass into clean-burning fuel stripped of impurities such as mercury, sulfur and carbon dioxide. Most of the flight tests have used natural gas, but Air Force officials think that their long-term energy strategy lies in liquefied coal, because the fossil fuel is so abundant in the United States.

The U.S. has 27 percent of the world’s coal supply – 493 billion tons – and sometimes is referred to as “the Saudi Arabia of coal.”

Despite its availability, however, coal seldom has been seriously considered as an alternative energy source because converting it to liquid is so expensive. However, liquid coal is getting a fresh look as crude oil prices soar past $100 a barrel.

While coal-to-liquid advocates say that the conversion process will result in an ecologically clean product, many environmental groups and their supporters in Congress think that expanding the use of coal will worsen carbon emissions and global warming.

“I think across the board there is going to be opposition from the environmental movement,” said John C. Topping, the president of the Climate Institute in Washington. “I’d say it’s going to be almost universal because of the climate concerns.”