The White House continues to hold out hope that the "final, fully verified denuclearization" of North Korea is possible by the end of President Donald Trump's term in 2021, a senior administration official said Thursday, amid reports warning that a key rocket site is now "operational" despite a promise by Pyongyang to dismantle it.

"We still believe this is all achievable within the president's first term, and that's the timetable we're working on," the senior U.S. State Department official told reporters on condition of anonymity. "We have discussed extensively the outlines of the calendar that allow us to do that, and it is doable."

The official also threw cold water on a more incremental process, despite earlier allusions to the U.S. being open to such an approach.