It also prompted North Korea's military to announce that it is considering an attack on Guam, a USA island territory that hosts about 6,000 American troops.
Trump and North Korea have been exchanging escalating threats, with the North saying it was examining plans for attacking Guam. "They will be met with fire and fury like the world has never seen", warned Trump.
North Korea has made no secret of its plans to develop a nuclear-tipped missile able to strike the United States and has ignored all calls to halt its weapons programs.
North Korea has said it may attack Guam in retaliation.
"Donald Trump, it seems to me, is growing ever more irrational and belligerent, shoot from the hip, and we have to bear in mind that in the context of these North Korean remarks that the president has nuclear weapons in his holster", Humphrey said.
Trump amplified his inflammatory rhetoric toward Pyongyang this week, telling reporters on Tuesday that North Korea would face "fire and fury" if the country continued to threaten the U.S.
North Korea said it was "carefully examining" a plan to strike Guam, which is home to about 163,000 people and a USA military base that includes a submarine squadron, an airbase and a Coast Guard group. "In fairness, there has not been time for such plans to have been prepared, reviewed and implemented, so this is not a surprise", he said.
"I think this reflects that the President is not well-informed about the capacity of the USA nuclear arsenal and its capabilities", said Reif, who called the tweets "reckless posturing". While the review may recommend changes, as a 2010 review by Mr. Obama did, it has yet to be completed. Lindsey Graham says Trump isn't going to just "contain the threat".
China, North Korea's closest ally despite Beijing's anger at Pyongyang's missile and nuclear programmes, described the situation as "complex and sensitive", and urged calm and a return to talks.
She noted more than 200,000 Americans lived in the region and that 26 million people reside in Seoul, the South Korean capital.