Sarin gas is in the news, but it's not especially useful as a material for making missile warheads.   The bigger danger is that North Korea would make a "dirty" nuclear warhead.  The warhead would not cause a nuclear explosion, but it would contaminate large areas with deadly levels of radioactivity.  Depending on the radioactive material used, the area could be contaminated and uninhabitable for many decades.  But in view of Japan having a nuclear accident at their Fukushima nuclear power plant (three reactor meltdowns), it might not be "politically correct" in Japan to talk about radioactive contamination.