President Trump's latest tweet poking fun at North Korean leader Kim Jong Un is "going to really upset" him, said former U.S. Ambassador to the United Nations Bill Richardson.

He was reacting to Trump's snarky tweet, sent during his tour through Asia: "Why would Kim Jong-un insult me by calling me 'old,' when I would NEVER call him 'short and fat?' Oh well, I try so hard to be his friend - and maybe someday that will happen!"

"I think so far the North Korea aspect of the trip was going decently until this tweet," Richardson said to MSNBC's Kasie Hunt on her show Sunday evening.

"Yes, he's insulted the president, too. But the North Koreans are very sensitive to these kind of insults," added Richardson, who is also a former Democratic governor of New Mexico.

The North Korean regime recently called Trump a "lunatic old man" that is escalating tensions by meeting with Asian leaders. The two leaders have been quick to name-call one another since Trump took office. Kim had previously called Trump a "Dotard" and Trump frequently refers to Kim as "Rocket Man" or "Little Rocket Man."

Richardson went on to speak from his experience working in North Korea after it was revealed former President George W. Bush called Kim's father and predecessor, Kim Jong Il, a "pygmy." During the interview Richardson said Bush used the word "dwarf."

"I just never stopped hearing about it," said Richardson, who has been involved in multiple diplomatic engagements with Pyongyang.

Richardson explained that diplomacy is "very traditional. Especially in Asia" and stressed that the U.S. should "focus on finding a diplomatic solution, not insulting."

Trump did say, during a joint press conference in Vietnam hours after that tweet, he prefers "progress, not provocation" in dealing with North Korea, which continues to develop its nuclear weapons and missile programs despite international sanctions aimed at getting Pyongyang to stop.