Senior U.S. military officials say North Korea's ballistic missiles could threaten the west coast of the United States in three to five years.

The vice chairman of the Joint Chiefs of Staff, Lieutenant General Patrick Cartwright, said North Korea's three tests of its longest-range missile, though unsuccessful, have demonstrated an increasing capability. 

His comments came during a hearing Tuesday of the Senate Armed Services Committee.

Deputy Defense Secretary William Lynn told lawmakers that North Korea's missile testing program has accelerated.  Lynn said the future threat of Pyongyang's long-range missiles provides a strong reason for the United States to maintain ground-based missile interceptors.

The head of the U.S. military's Missile Defense Agency, Lieutenant General Patrick O'Reilly, said North Korea is involved in sharing technology with countries including Iran, but would not specify other countries.  O'Reilly said there is an extensive effort to sell North Korean technology and hardware.

Some information for this report was provided by AP and Reuters.