https://www.reuters.com/article/us-northkorea-missiles-tillerson/tillerson-says-u-s-ready-to-talk-to-north-korea-japan-wants-pressure-idUSKBN1E62UI?il=0

"Tillerson also disclosed the United States had been talking to China about how to secure North Korea’s nuclear weapons in the event of a collapse of the government in Pyongyang. He said Beijing had been given assurances that if U.S. forces had to cross into North Korea they would pull back across the border into the South."