seoul u.s. president donald trump tell south korea saturday explain china president xi jinping america position deployment anti-missile defense system south korea accord statement south korea act leader trump inform south korean prime minister hwang kyo-ahn saturday telephone call summit discussion xi china oppose deployment u.s.-supplied high altitude area defense thaad south korea argue could threaten security south korea unite state say aim counter north korea missile threat china increase pressure impose restrictions south korean company business china lead many south believe beijing retaliate deployment missile system president trump say president xi hold in-depth discussions seriousness north korea nuclear problem respond also say convey u.s. position thaad deployment statement say trump press xi curb north korea nuclear programme summit meet trump mar-a-lago resort florida u.s. secretary state rex tillerson say u.s. military begin deployment thaad system march day north test-fired four ballistic missiles china directly say target south korean company trump explain u.s. position thaad xi south korea