South Korea put it's armed forces on alert, feeling that North Korea was planning another military move to increase the pressure on the US to give in to the north's extortion demands.
South Korea has urged the north to dismantle it's nuclear weapons program, and not try to extort economic aid and a non-aggression treaty from the US in return for this. The north has proposed such a deal, but without inspections to insure that the north complied. The south fears that the north will try some crazy stunts to pressure the US into giving in. At the moment, no decision has been made by America, and it seems likely that North Korea's demands will not be met. There appears to be a lot of enthusiasm for an aggressive response, like a naval blockade, to insure that no nuclear weapons or missiles are exported. But this sort of thing is considered an act of war and the north could respond by bombarding Seoul, South Korea's largest city.
American intelligence experts believe that increased activity at North Korea's nuclear fuel processing facilities indicates an effort to produce nuclear fuel for more atomic bombs.
Japanese media reported that in 1993, after North Korea test fired a ballistic missile that flew over Japan, the Japanese government ordered a study of military options against North Korea. The study concluded that Japanese armed forces could attack North Korean missile bases, using air power and ground forces, but Japanese losses would be high.