The Chinese would be much more concerned than the Russians. The Russo-Korean border is only a few kilometers long, and I don't think that the NATO will install bases anyway near that narrow strip of land. On the other hand, the Sino-Korean border runs through three strategically important provinces of China.
Indeed, China would be quite upset to find that the US and SK armies started building bases next to their borders.
Only a few kilometers long you said. Well for example the Isthmus of Perekop was also only a few kilometres wide, Mannstein however somehow managed to push through a fairly big army on it, and Vladivostok not much farther from the NK border than Sevastopol was from Perekop. So I don't think the Russians would be happy bunnies.
Remember the Korean war of the 1950s? There could be a repetition of this. The Chinese will provide the manpower and the Russians will provide equipment for the North Koreans. The Americans will fight it out for many years, before pulling out with heavy casualty figures.