North Korea says it has developed intercontinental missiles capable of targeting any place in the United States.

Now comes the hard part of fulfilling the declared goal of its leader Kim Jong Un: perfecting a nuclear device small and light enough to fit on the missile without affecting its range as well as making it capable of surviving re-entry into the earth's atmosphere.

To do that, weapons experts say, the isolated state needs to carry out at least another nuclear test, its sixth, and more tests of long-range missiles.