WASHINGTON, April 28 - The head of the Defense Intelligence Agency said today that American intelligence agencies believe North Korea has mastered the technology for mounting a nuclear warhead on its missiles, an assessment that, if correct, means the country could build weapons to threaten Japan and perhaps the western United States. The conclusion was part of a total reassessment of North Korea's capabilities that the D.I.A.'s chief, Vice Adm. Lowell E. Jacoby, said was still under way. While Admiral Jacoby said North Korea was judged to have the capability to put a nuclear weapon atop its missiles, he stopped...