washington unite state concern north korea might able deter senior trump administration official say thursday draw distinction washington view pyongyang model deal former cold war foes official speak reporters condition anonymity also say grave risk north korea might miscalculate u.s. response behavior warn pyongyang under-estimate american protect ally u.s. experts continue assess north korean nuclear test sixth powerful official information contradict pyongyang assertion detonate hydrogen bomb u.s. unsure north korea deter trump administration official