This is something that U.S. officials, as well as many in the mainstream press, have lamented and complained about for years. Apparently it's not enough that the North Korean people already suffer from extreme poverty, as a result of both their socialist economic system and decades of brutal sanctions. If those horrible Chinese would only cooperate, the North Koreans could be squeezed even more, perhaps even with mass starvation.
Let's get one thing straight: The UN is just a cover. The sanctions are orchestrated, engineered, and enforced by the U.S. government. The idea of using the UN for cover is so that it will appear like it's an international act, rather than an act of the U.S. government. Everyone knows that any member of the United Nations that refuses to vote with the Empire will suffer the wrath of the Empire, either with a cutoff of U.S. foreign aid or even a regime-change operation against it.