They have been working on achieving three major items: successful re-entry of a missile. minaturizing a warhead for the missile and achieving a long range missile (which would be capable of hitting mainland U.S.). They achieved the first goal last week, wth successful re-entry of an intermediate range missile.
Via BBC:
North Korea has conducted another missile test, South Korea’s military has said.
South Korea’s Office of Joint Chiefs of Staff said the projectile was launched on Sunday afternoon local time.
The test comes a week after North Korea tested another missile, which it said was a new type of rocket capable of carrying a large nuclear warhead.Last Monday, the UN Security Council again demanded that Pyongyang conduct no further such tests.
It stressed the importance of North Korea “immediately showing sincere commitment to denuclearization through concrete action”.