and nobody knows how north korea's erratic leader will now react. >> this particular tweet is in essence telling the north koreans, putting them on notice that they are going to be watched very carefully by the incoming administration and that they don't have cart blanche. >> reporter: this after kim said -- >> translator: cutting-edge arms equipment is actively progressing and intercontinental ballistic missile test launch preparation is in its last stage. >> reporter: it's not clear how soon kim can be ready to launch a missile that could reach the u.s. but the prospect raises alarm. if a nuclear weapon exploded over a west coast population center like los angeles or san francisco, tens of thousands could be killed. even a non-nuclear north korean attack into south korea could also kill tens of thousands including 30,000 u.s. troops based there. trump on the campaign trail was