The threat from the North Koreans came ahead of a unanimous vote in the U.N. Security Council approving its toughest sanctions yet on the North in response to an atomic test last month.
North Korea has escalated its bellicose statements this week as the tightening of U.N. sanctions loomed. It has also threatened to scrap the cease-fire that ended the 1950-53 Korean War.
"I can tell you that the United States is fully capable of defending against any North Korean ballistic missile attack," said White House spokesman Jay Carney.
North Korea has now conducted three nuclear tests. In the past year, it has made strides toward its goal of having a nuclear weapon that could threaten the U.S. although experts doubt it yet has the capability to hit the U.S. with a ballistic missile or miniaturize a nuclear device to mount on such a missile.
However, the North possesses hundreds of shorter-range missiles that could hit U.S. bases in Japan and South Korea, said Victor Cha, Korea chair at the Center for Strategic and International Studies think tank.
It is difficult to know how capable U.S. missile defense is, should it be required.
Carney alluded to the development of U.S. system designed to defend against long-range missiles. He said the U.S. is on a "good trajectory" after success in its return to testing of the Ground-Based Interceptor.