The United States Pentagon chief vowed on Thursday to "remain fully ready" to respond to any threats from North Korea, naming the country in an informal shortlist of problem regions that Washington will keep an eye on.

In a written memorandum to all U.S. Department of Defense employees, Secretary of Defense Lloyd Austin singled out China as the "number one pacing challenge" that will require the U.S. to "bolster deterrence" and "maintain its competitive advantage." However, Austin also said mentioned North Korea as an "advanced and persistent threat."