"We are still recovering from September 11," he said in response to a question, referring to the impact on the airline industry of the 2001 hijacked aircraft attacks in the United States.

"Aviation is growing in some sectors of the world. It's not particularly growing in the United States," he said, adding that many airlines in the United States had been "teetering on the edge of bankruptcy".

The UN's Intergovernmental Panel on Climate Change estimates that aviation causes 3.5 percent of global warming, widely blamed on human activities, and that the figure could rise to 15 percent by 2050.

In contrast with Watson, the European Commission expressed a willingness to include aviation, perhaps by imposing taxes or charges on aviation fuel or by widening the trading of emissions of heat-trapping gases like carbon dioxide.

"We believe that we need to tackle aviation emissions also in the future climate policy review after 2012," said Artur Runge-Metzger, head of the Commission's climate, ozone and energy unit.

The European Commission plans to issue a report in June or July about ways to reduce aviation's impact on the climate, he said.

Aviation is currently outside the UN's Kyoto protocol, which seeks to cut developed nations' emissions of carbon dioxide from factories, power plants and vehicles by 5.2 percent below 1990 levels by 2008-12.

President George W. Bush pulled out of Kyoto in 2001, saying it was too expensive and wrongly excluded developing nations.

The European Union says that its total greenhouse gas emissions fell by 3 percent from 1990 to 2002 while emissions from international aviation rose by almost 70 percent.