Yesterday Colorado Congressman Doug Lamborn disclosed a previously classified Defense Intelligence Agency (DIA) evaluation that North Korea could put a nuclear warhead on a missile, according to the New York Times. When I worked at the State Department, including in its Bureau of Intelligence and Research, people uniformly thought that DIA's intelligence analysis was poor, except in areas such as particular tactical weapons evaluations. In strategic areas, such as nuclear weapons development, DIA always tended to overplay the threat, presumable because it meant budget money. The Pentagon needed dire threats to justify spending the huge amounts of money it wanted for its various weapons programs. Thus, it needed to build up the threatening image of the enemy, whoever it was, the old Soviets, or the new terrorists, or North Korea.
I think there probably was some collusion between Congressman Lamborn and the Pentagon. It may not just be accidental that the sentence or paragraph that Lamborn quoted was unclassified, while the rest of the report was. Somebody at DIA probably wanted to get that analysis out, and worked out a way to do it through Lamborn. But the rest of the US Government has pretty much disavowed the statement as just the unfounded opinion some crazy DIA analysts.
I don't think that even next door neighbor South Korea needs to worry about being hit by a nuclear tipped North Korean missile, although it might need to worry about a nuclear weapons delivered by some more conventional means, such as aircraft, truck or ship. In addition, North Korea probably has few nuclear weapons. Despite their flouting restrictions on their nuclear program, over the years the international pressure has slowed down their program, meaning that they have relatively little nuclear material, either plutonium or enriched uranium. Just recently they have threatened to restart the plutonium production reactor which has been shut down for years.