The officials said the Chinese Peoples' Liberation Army was increasing troop numbers in apparent preparation for a possible influx of refugees due to instability, or regime collapse, in North Korea.
US and South Korean intelligence agencies think Mr Kim suffered a stroke in August that left him paralysed on his left side, possibly severely enough to prevent him from walking. While Washington believes Mr Kim is still in control, there is concern about how long he can remain so given his illness.
One US official cautioned that the increase in Chinese troops was not “dramatic”, but said China was also building more fences at key locations.