WASHINGTON, March 7 — The North Korean fighter jets that intercepted an unarmed American spy plane over the Sea of Japan last weekend were trying to force the aircraft to land in North Korea and take its crew hostage, a senior defense official said today. One of the four North Korean MIG's came within 50 feet of the American plane, an Air Force RC-135S Cobra Ball aircraft, and the pilot made internationally recognized hand signals to the American flight crew to follow him, presumably back to his home base, the official said. The American crew members ignored the gesture commands,...