"If anything, maybe that statement wasn't tough enough," he told reporters at his New Jersey golf club.
When asked what could be tougher than "fire and fury," the president responded: "we'll see." He also did not comment on whether the U.S. is considering a pre-emptive strike on North Korea.
"The people of this country should be very comfortable, and I will tell you this: If North Korea does anything in terms of even thinking about attack, of anybody that we love or we represent or our allies or us, they can be very, very nervous," Trump said. "I'll tell you why, and they should be very nervous. Because things will happen to them like they never thought possible."
The remarks continue a series of aggressive exchanges between Trump and the North Korean regime in rhetoric more forceful than U.S. presidents typically use. As tensions escalate and Pyongyang keeps developing its nuclear and missile programs in the face of international opposition, Trump and other world leaders face a set of limited and difficult options to respond.
On Tuesday, Trump strongly warned Pyongyang against threatening the United States in a reportedly improvised line.
"North Korea best not make any more threats to the United States," Trump told reporters, speaking slowly and deliberately with his arms crossed in front of him. "They will be met with fire and fury like the world has never seen. He has been very threatening ... and as I said they will be met with fire, fury and frankly power, the likes of which this world has never seen before."
North Korea's state media responded by saying the country was considering a plan to attack the U.S. territory of Guam. It also called Trump's statement "nonsense" and said only "absolute force" can work on him.
"Let's see what he does with Guam. If he does something in Guam, it will be an event the likes of which nobody's seen before, what will happen in North Korea," Trump told reporters Thursday. He added that his comments on Guam were not a "dare," just a "statement of fact."
Trump's comment followed reports that Pyongyang had successfully created a miniaturized nuclear weapon. It marks a major step in the country's nuclear ambitions.
North Korea has continued its aggression and missile tests in the face of economic sanctions. The most recent round was unanimously approved by the U.N. Security Council on Saturday.
Trump also contended "there are no mixed messages" coming out of his administration on North Korea. Some aides had reportedly said not to read too far into the president's "fire and fury" comments.
"I think what the president was just reaffirming is that the United States has the capability to fully defend itself from any attack, and our allies, and we will do so," Secretary of State Rex Tillerson said Tuesday.
Trump added that he believes the latest sanctions on North Korea may not accomplish as much as some would hope. China, Pyongyang's only major ally, "can" and "will" do a lot more to keep the isolated regime in check, the president said.
Trump said he wants to "de-nuke the world."