The buildup likely serves two goals, the officials said. One is to bolster border security in case a conflict sends large numbers of refugees from the impoverished state into China.

Additionally, the troop buildup is a signal to Pyongyang that China will abide by its defense commitment to North Korea in the event of renewed conflict.

China’s military maintains a mutual defense treaty with North Korea. The last time Chinese troops defended North Korea was during the Korean War.

U.S. officials also said there were signs of increased movement inside North Korea, specifically movement of road-mobile missile systems. One official said activity was seen at the long-range missile launch complex at Tongchang-ri on the west coast.