North Korea escalated nuclear tensions on Friday by declaring it has solved "all the technological matters" involved in using plutonium extracted from nuclear fuel rods to build atomic bombs. The communist state said a day earlier it had completed reprocessing the rods and had started using plutonium to make nuclear weapons as a deterrent against what it calls a U.S. plan to invade. Washington says it has no intention of invading and Pyongyang's nuclear ambitions are a "serious concern" to the international community.