In the wake of the pageantry surrounding U.S. President Donald Trump's historic summit with North Korean leader Kim Jong Un, questions over the substance of a vaguely written denuclearization deal lingered as some experts and former officials anticipated how the White House might beef up the bare-bones agreement.

Trump on Wednesday thanked Kim in a series of tweets "for taking the first bold step toward a bright new future for his people" a day earlier, writing that "The World has taken a big step back from potential Nuclear catastrophe!"

The words of praise stood in stark contrast with earlier threats of "fire and fury" by the mercurial U.S. president and came on the heels of a joint statement a day earlier that Trump hailed as a "comprehensive document" that would kick-start "the beginning of an arduous process."