As reported by the New York Times, Donald J. Trump warned Chinese President Xi that the U.S. could strike out alone in pressuring North Korea, after yesterday's missile launch that many analysts fear was a successful test of an Intercontinental Ballistic Missile (ICBM), capable of reaching Alaska.
The U.S., China, and North Korea each have nuclear weapons arsenals (although the U.S.'s dwarfs the other two, especially North Korea's), and itself regularly has tested ICBMs for decades (as from Vandenberg Air Force Base in CA, to the Marshall Islands).