The talks between Pyongyang and Seoul have progressed considerably faster than the ongoing negotiations between Pyongyang and Washington. Whilst both the US and South Korea have insisted that the alliance between them is solid, reports suggest that US officials are concerned that things are developing at an alarming pace between the Koreas, reducing the pressure on Pyongyang to denuclearise in the near future. Calls for sanctions reduction have risen as the result of numerous symbolic actions taken by Pyongyang during inter-korean summits. These include recently reconnecting roads and rail links, closer economic and political ties between the two countries (despite economic sanctions) and social/cultural developments including Kim Jong Un stating that he intends to visit Seoul in the future and recent commitments to more family reunions between Koreans.
The United Nations recently permitted Seoul and Pyongyang to push forward with studying the practicality of linking their rail networks across the border. News regarding developments in denuclearisation has slowed in recent months with the most recent development being the cancelled meeting in New York between Kim Yong Chol and Mike Pompeo, a sign that talks were sliding off the rails. The best thing moving forward for the US-ROK alliance would be to organise high level meetings between the governments to finalise a joint strategy that encompasses both the interests of Washington and Seoul to ensure that one side does not outrun the other.