Despite a "disappointing" response from Pyongyang over prospects for diplomacy with the U.S., Washington has "got to keep pressing forward" on a deal with the North before the end of the year, Secretary of Defense Mark Esper told reporters on Thursday.

Esper's comments, made at the end of an official visit to Seoul, appeared to channel the same optimistic outlook that the U.S. special representative to North Korea, Stephen Biegun, had presented to a panel of Senators one day before, when he told lawmakers that "the window is still open" for an agreement.