seoul u.s. defense secretary jim mattis say saturday could imagine unite state ever accept nuclear north korea warn rapidly advance nuclear missile program would undermine strengthen security mattis pain week-long trip asia stress diplomacy america prefer course message return top-level military talk seoul saturday tense border area north korea friday still warn pyongyang military match u.s.-south korean alliance diplomacy effective back credible military force make mistake attack unite state ally defeat use nuclear weapons north meet massive military response effective overwhelm mattis say mattis south korean counterpart defense minister song young-moo dismiss idea deploy tactical nuclear weapons peninsula response north advance u.s.-korean alliance ability respond even event nuclear attack north song add tension north korea unite state build series nuclear missile test pyongyang bellicose verbal exchange north korean leader kim jong un president donald trump cia say north korea could months away develop ability hit unite state nuclear weapons scenario trump vow prevent mattis say kim behavior leave room imagine accept pyongyang nuclear status imagine condition unite state accept north korea nuclear power mattis tell news conference trump threaten destroy north necessary leave first trip president asia next week include stop south korea meet president moon jae-in moon talk mattis friday say aggressive deployment u.s. strategic assets region include overflights u.s. bombers effective deter north korean threat u.s. intelligence experts say pyongyang believe need nuclear weapons ensure survival skeptical diplomatic efforts focus sanction get pyongyang denuclearize mattis suggest however pyongyang need understand weapons program would strengthen defenses north say want nuclear-tipped missile capable reach unite state remain current path ballistic missiles atomic bomb counter-productive mattis say add north korea would reduce security still attempt force north denuclearize could devastate consequences thank part large amount artillery train seoul mattis trip inter-korean demilitarize zone dmz friday brief posture north korean artillery defense minister song suggest u.s. south korean missile defenses simply could stop weapons train south defend many lras long-range artillery infeasible opinion song tell mattis dmz cite need strategies offensively neutralize artillery event conflict mattis reply understand still mattis reaffirm reporters south korea military options exist deal north korean threat spare south capital seoul mattis make assurances past disclose options mattis seoul say u.s. ca n't accept nuclear north korea