One of the largest environmental groups in the United States wants to ground the growing use by American and Canadian airlines of fuel derived from Alberta's oilsands.

In letters being sent Thursday to 15 airlines, including Air Canada (TSX:AC.A) and WestJet (TSX:WJA.A), the Natural Resources Defence Council says their growing dependence on "dirty fuels'' should be reduced by conservation measures and tighter fuel efficiency standards.

"There is no reason for airlines to be using fuel that results in the destruction of Canada's biologically and incredibly carbon-rich boreal forest,'' Liz Barrat-Brown, a council lawyer, said in an interview from Washington on Wednesday.

"We don't want to be part of that problem. We're asking the airlines to take a position that they are not part of creating the destruction of what we believe to be an incredibly important Canadian national resource.''

Barrat-Brown said air travel is the fastest-growing transport sector.

The letter is the latest effort by the group, considered one of the largest and most influential American environmental lobby groups, to attack oilsands development by trying to persuade customers that the costs are too high.

Barrat-Brown said about 15 per cent of the refined product from the oilsands ends up as jet fuel and most of that is used by commercial aircraft. Major U.S. airports in Chicago and Denver have direct links to refineries making fuel from oilsands oil. United Airlines and American Airlines have both said they support expanding the pipeline bringing such oil to the Chicago region.

The council's letter to Air Canada CEO Montie Brewer challenges him to "publicly oppose the expansion of dirty fuels in North America and use of these fuels by aircraft, fleets and other vehicles.''

Barrat-Brown suggested adopting fuel conservation methods already used in other countries, such as more direct airplane routing and more gradual descents, could reduce the need for airlines to use oilsands-derived fuel.

The council believes the U.S. should pass regulations to discourage the use of such fuel, which generates up to three times as much greenhouse gas as conventional oil during its production and use.

As well, she pointed out that aviation experts predict that 80 per cent of the airplanes now flying will be replaced by 2026, so carriers should buy new, more efficient replacements.

But in a report released last October the Air Transport Association of Canada said that aviation contributed only 1.2 per cent of Canada's greenhouse gas emissions in 2005. The report also said the amount of fuel used per passenger-mile was expected to decrease 30 per cent by 2012 -- although the absolute amount of fuel and emissions would go up due to an increase in flights.

"Aviation represents an environmentally responsible choice for travelling between many Canadian cities in comparison with using motor vehicles or railways,'' the report says.

Oilsands oil is also under attack in Europe, where last year the World Wildlife Fund in the United Kingdom released a report blaming oilsands expansion for Canada's failure to live up to its Kyoto commitments.

Environmentalists have gone after the customers of the oilsands before.

Last summer, the council held a news conference in Washington to call the Alberta oilsands "bottom-of-the-barrel'' energy -- criticism that drew a vigorous response from Premier Ed Stelmach.