I guess, neither big countries want the unification of North Korea and South Korea. If any external force tries to intervene in Korea peninsula and change current situation, Korea peninsula might become another balkan peninsula, fueling another world war.
The US government is aimed to press Beijing to exert more pressure on North Korea. But I doubt the North Korea is still “under control of” its big brother — Beijing. How effective economic transaction from China would be working?
http://time.com/4700527/rex-tillerson-asia-china-japan-south-north-korea/