About two weeks ago, in anticipation of the tests, the North American Aerospace Defense Command in Colorado was put on heightened alert, or "Bravo-Plus" - a status slightly higher than a medium threat level. NORAD and the US Northern Command are responsible for defending US territory.

However, the missile firings - launched within minutes of Tuesday's liftoff of the shuttle Discovery - were not viewed as an eminent threat against America.

"It's very difficult to know what the North Koreans think they are doing this for," Hadley said. "Obviously, it is a bit of an effort to get attention, perhaps because so much attention has been focused on the Iranians."

Michael Green, Bush's senior adviser on Asia until he stepped down in December to work for a think tank, said he's not surprised that the Taepondong-2 failed because it has not undergone comprehensive testing. He said it showed North Korea's President Kim Jong Il's diplomatic and technical ineptitude.

"This is not a good day for the dear leader," Green said.