On the other hand Trump expressed his wish to befriend with North Korean leader.
"They really want to know what is his end game", said DiMaggio, a scholar at New America who specializes in talking with rogue regimes and has spent almost two decades in secret discussions with the North Koreans.
It's odd, one minute Kim is talking about nuclear war, testing missiles, then he calls Trump "old".
North Korean officials "question his erratic behavior, and also his mounting problems here at home, with the investigation being conducted by Robert Mueller", DiMaggio told The Global Politico.
Visiting Asia, the president has tried to bring the world together in pressurising North Korea's nuclear weapons programme, something that's proving somewhat of a tough task.More news: At Least Three Dead, Students Wounded After Shooting At California School
North Korea is not known to have tested any of its missiles or nuclear devices since September 15, a relative lull after a brisk series of tests earlier this year.
On a 11-day diplomatic trip through Asia this month, Trump alternated between hinting at nuclear negotiations and tweeting sarcastic outrage about Kim calling him "old".
There has been speculation in both North Korea and the U.S. that Trump may be feigning unpredictability, emulating former United States president Richard Nixon's "Madman Theory" -whereby enemy countries were tricked into avoiding conflict with the USA, believing Nixon would be irrational and unpredictable in his response.
In a response to North Korea calling Trump's speech in South Korea "reckless remarks by an old lunatic", Trump tweeted from Hanoi on Sunday morning: "Why would Kim Jong-un insult me by calling me 'old, ' when I would NEVER call him 'short and fat?'" She said Trump has broken America's golden rule of negotiating with Kim: "Whatever you do, don't personally insult this man".