When Donald Trump becomes U.S. president next month, one issue above all others could force his new administration to work closely with China and underscore why he and Beijing need each other — North Korea.

A nuclear armed North Korea, developing missiles that could hit the U.S. West Coast, is clearly bad news for Washington but also Pyongyang's sometimes-reluctant ally Beijing, which fears one day those missiles could be aimed at them.

"There is enormous space for the two countries to cooperate on North Korea. The two must cooperate here. If they don't, then there will be no resolution to the North Korean nuclear issue," said Ruan Zongze, a former Chinese diplomat now with the China Institute of International Studies, a think tank affiliated with the Foreign Ministry.