"This man will not get away with what he is doing," Trump told reporters. "If he utters one threat in the form of an overt threat ... or if he does anything with respect to Guam, or any place else that is an American territory or an America ally, he will truly regret it and he will regret it fast."
Later, Trump told reporters that Guam is "very safe," and that there would be "big trouble" in North Korea if the country were to attack the US territory.
Trump's 17-day working vacation in suburban New Jersey has been dominated by an uptick in rhetoric with North Korea, with Trump first promising "fire and fury" if the rouge nation continued to threaten the United States. Trump stood by that statement on Thursday, saying that it may have not been tough enough.
On Friday, during a meeting on workforce development at his private golf club in Bedminster, New Jersey, Trump reiterated his sharp words against the communist country.
"I hope they are going to fully understand the gravity of what I said. And what I said is what I mean," Trump said, adding that his statements' significance was "pretty obvious" and his administration is looking "very carefully" at possible future actions against North Korea.
In response to Trump's comments, North Korean officials have indicated they plan to strike near Guam with missiles by the end of this month, a threat that has ratcheted up concerns within the Trump administration.
Trump also blamed his predecessors for discord in the Korean peninsula, saying that the current rhetoric is an extension of past failures and he has no choice but to respond to the rogue nation.
"We want to talk about a country that has misbehaved for many, many years, decades actually, through numerous administrations and they didn't want to take on the issue," he said. "I have no choice to take it on -- and I am taking it on -- and we will either be very, very successfully quickly or we are going to be very, very successful in a different way, quickly."
Foreign policy and national security experts have raised questions about Trump's threatening response to North Korea, noting that past administrations have tried to avoid the President commenting on North Korea's threats in order to avoid giving them credibility.
"I take exception to the President's comments because you've got to be sure that you can do what you say you're going to do," Sen. John McCain, an Arizona Republican, said in an interview with Phoenix radio station KTAR earlier this week. "The great leaders I've seen don't threaten unless they're ready to act and I'm not sure President Trump is ready to act."
Trump dismissed the criticism saying the only reason they are critical of his response is "because it's me."
"If somebody else uttered the exact same words that I uttered they would say what a great statement, what a wonderful statement," Trump said, adding that there are "tens of millions of people in this country that are so happy with what I am saying."
Trump's rhetoric and North Korea's response have unsettled powers in the Asia-Pacific region, with some scrambling to respond to the escalation of tensions.
Neighboring powers urged restraint. Russia called on the Trump administration to keep calm, and on Friday Russian Foreign Minister Sergey Lavrov told state media that his country was working with China on a "double freezing" plan to address the threat.
"Kim Jong Un freezes any nuclear tests and any missile launches -- we mean ballistic missiles -- and the US and South Korea freeze large scale military exercises," Lavrov said, adding that North Korea regularly uses the drills to justify its missile tests and nuclear program.
Trump will discuss North Korea further on Friday afternoon when he meets with US Ambassador to the United Nations Nikki Haley and Secretary of State Rex Tillerson.