SEOUL - South Korea's defence ministry said on Friday it did not believe North Korea had succeeded in miniaturizing a nuclear warhead for a missile, casting doubt over a fresh US intelligence assessment of the reclusive state's nuclear capability."Our military's assessment is that the North has not yet miniaturized," ministry spokesman Kim Min-seok told a news briefing."North Korea has conducted three nuclear tests but there is doubt whether it is at the stage where they can reduce the weight and miniaturize to mount on a missile."