The US is opposed to direct South Korea and North Korea negotiations. The only kind of negotiation it approves of is multi-party talks between China, Japan, and South Korea. It does not want to commit to anything except to enforce the multi-party agreement. 

Well that won't work. Our allies say that the US should enter into direct negotiations with the North Koreans because it is the US that threatens the existence of North Korea. 

The South Koreans don't want to start another civil war and the Japanese Constitution doesn't allow it to go to war while North Korea is an ally of China.

By the way over 50 years after the Korean War the US and North Korea still has pro-forma meetings to discuss ending the armistice truce established 50 years ago. That hasn't gone anywhere. The US probably doesn't want to negotiate because it never won that war. It wanted N  Korea to surrender.