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North Korea, which this month threatened to carry out a fourth nuclear test, may be closer than previously thought to mounting a nuclear warhead on a missile, some experts say, making a mockery of years of U.N. sanctions aimed at curbing its efforts to obtain nuclear weapons. North Korea has long boasted of making strides in acquiring a “nuclear deterrent,” but there has been general skepticism that it can master the step of miniaturizing a nuclear warhead to fit on a ballistic missile. No one outside the inner circle of North Korea’s nuclear program likely knows what advances the country has made. But there has been a shift in thinking by some who study the North since it conducted a nuclear test in February last year and amid on-off indications that it is preparing another. More