While South Korea’s President Moon is eager to continue making progress in peace talks with North Korea, his government is increasingly split over what they can get away with doing, and how much they have to get America’s permission for.
The Trump Administration has made no bones of objecting to some of those matters. The US in particular didn’t like reports of restoring road and rail links with North Korea, though it didn’t stop South Korea from doing so.
The big issue right now is a liaison office, which would serve as a de facto base of diplomatic operations for South Korean officials in North Korea. South Korea’s Unification Minister warns that the US may not like that.
The issue is providing electricity to the office, which would be in North Korea, and heating the office in the winter. Both of those would arguably be violating US sanctions on providing energy to North Korea.