While he did not refer specifically to the rocket-engine test, he said Kim Jong Un was "acting very, very badly".
Trump made the comments to reporters while departing leaving his Mar-a-Lago resort in Florida, where he spends nearly every weekend.
North Korea on Monday blasted the US administration for pressurising Pyongyang with threatening remarks when US Secretary of State Rex Tillerson visited Asia recently.
"I won't deny that there are a few other North Koreans involved in the Kim Jong Nam murder and we will use legal channels to access them", Tan Sri Khalid was quoted as saying by state news agency Bernama. He had been known to speak out publicly against his family's dynastic control of North Korea.
"The world will soon witness what eventful significance" its most recent rocket engine test would have, the report added.
Possession of an intercontinental ballistic missile could allow North Korea to threaten the continental United States.
"If we recognize the North as a nuclear state, we are providing clear justification for its nuclear and missile tests and admitting its claim that it is developing a nuclear arsenal as a result of external threats [from the USA and South Korea]", explained the diplomat, who defected to Seoul with his family in August 2016.
Judging from pictures released by the government over the weekend, the communist regime's dictator Kim Jong Un was all smiles as he watched his generals launch the high-thrust rocket engine from the Sohae launch station.
If the US were to take military action, potential targets could be North Korea's airfields in Hwangju, Kusong, and Wonsan provinces that have been used over the past year to test new Musudan and KN-11 missiles. North Korea's latest rocket engine test showed "meaningful progress" in its missile capabilities, Seoul said today, as the nuclear-armed state steps up its controversial weapons development programmes.
Sunday's test was a "precisely-timed spectacular", said Peter Hayes, director of the US-based Nautilus Institute, "the kind we've come to expect from Kim Jong Un". However, analysts have long accused Pyongyang of using the program as a covert weapons testing scheme.
Although neither side brought up the subject publicly, Tillerson was expected to raise the prospect of financial penalties on Chinese companies and banks that do business with close ally North Korea.
Pyongyang has been bristling over ongoing military drills involving American and South Korean troops along with Seoul's plan to deploy a US anti-missile system, THAAD. He's acting very, very badly.