Deputy Defense Secretary William Lynn agreed with an assessment by Sen. John McCain that the U.S. should be prepared for a "worst-case scenario" with North Korea.
Pentagon officials also told the Senate Armed Services Committee Tuesday that North Korea is working with other nations, including Iran and potentially Syria, to develop ballistic missiles.
President Barack Obama said Tuesday that a nuclear-armed North Korea poses a "grave threat" to the world and cannot achieve its own security by building illegal weapons.
Last week, Defense Secretary Robert Gates told lawmakers he was confident the United Statesfired toward America.
Gates said that the U.S. would have a "high probability of being able to defend ourselves" if the North fired a long-range missile. He added that U.S. missile interceptors "are fully adequate to protect us from the North Korean threat for a number of years."
Gates said that if the North Korean "threat" should change, the United States could easily expand its missile interceptor force.