North Korea's Kim Jong-un agreed Wednesday to shutter a missile-testing site and visit Seoul, a historic move the United States (US) welcomed by saying it was ready for immediate talks aimed at denuclearising North Korea by January 2021.Kim's ground-breaking announcement came at a summit with the South's President Moon Jae-in in Pyongyang, where the two signed a document to strengthen ties between the two halves of the divided peninsula.Progress on the key issue of the North&#0