Griffin, who has come under fire in Congress for cutting programs aimed at monitoring climate change, said in a US radio interview Thursday he had "no doubt that a trend of global warming exists."

But, he told National Public Radio, "I am not sure that it is fair to say that it is a problem we must wrestle with."

James Hansen, the National Aeronautics and Space Administration's top official on climate change, expressed shock at Griffin's comments in a later interview with NPR.

Asked his response, Hansen said: "I almost fell off my chair."

"It was a shocking statement because of the level of ignorance it indicated with regard to the current situation," Hansen told NPR.

"He seemed unaware that 170 nations agreed that climate change is a serious problem with enormous repercussions, and that many people will suffer if it is not addressed," he said.

Griffin said it was "NASA's responsibility to collect, analyze and release information," according to a statement issued later.

"It is not NASA's mission to make policy regarding possible climate change mitigation strategies," he added.

Bart Gordon, chairman of the Science and Technology committee in the Democratic-controlled House of Representatives, also took a shot at Griffin.

"Based on NASA's own five-year budget plan, the agency will be unable to start any of the new Earth observations initiatives recommended by the National Academies for the foreseeable future.

"That's not going to get us where we need to be in our understanding of climate change."

A national pro-environment group, the League of Conservation Voters, called on Griffin to resign.

Griffin's comments are "deeply troubling," said Gene Karpinski, the group's president.

"Global warming is happening, humans are contributing to the problem and we must take action to cut emissions of greenhouse gases that cause it -- this is not rocket science," Karpinski said.

Griffin's comments came on the same day US President George W. Bush said he would urge major industrialized nations at a summit next week to join a new global framework for fighting climate change after the Kyoto Protocol lapses.

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