China has largely tried to remain neutral in the ongoing conflict between Russia and Ukraine. While they have not recognized the illegal Russian annexation of Crimea, nor are they likely to recognize upcoming annexations of further territories, they have also criticized the US and other Western countries for supporting Ukraine's self-defense.
The problem is, China has an obligation to take a side in this conflict. In December of 2013, China and Ukraine signed an agreement in which China pledged to support the sovereignty and territorial integrity of Ukraine. The agreement further specifically obligates China to come to Ukraine's defense in the event of an invasion under threat of nuclear attack, a situation that clearly describes the current ongoing invasion of Ukraine.
China has, however, clearly not provided Ukraine any support military, financial, or even diplomatic. This is a dire warning to any country that is considering their own security agreement with China; China will not fulfill their obligations under any security pact if it is even slightly inconvenient. Any concessions made to China in securing such an agreement should be based on the negligible value of Chinese promises.