However, US Defense Secretary Jim Mattis later appeared to step back from the dire rhetoric of his boss. He described the prospect of war as "catastrophic" and added that diplomatic efforts to solve the North Korea nuclear crisis were yielding results.
Trump in his new remarks Thursday also warned North Korea it should be "very, very nervous" of the consequences if it even thinks of attacking US soil, after nuclear-armed Pyongyang said it was readying missile launch plans on the Pacific territory of Guam.
The Republican billionaire dismissed any criticism of his "fire and fury" warning of Tuesday, citing threats made by the regime of Kim Jong-Un to both Washington and its allies.
"It's about time that somebody stuck up for the people of this country and for the people of other countries. So if anything, maybe that statement wasn't tough enough," Trump said.
Trump also said China, Pyongyang's main diplomatic ally, could "do a lot more" to pressure Kim to end his country's nuclear weapons and ballistic missile programs.
Trump's comments, made from his golf club retreat in New Jersey, came after the North announced a detailed plan to send four missiles over Japan and towards Guam, where some 6,000 US soldiers are based.
Pyongyang said the scheme to target the island, a key US military outpost in the western Pacific, was intended to "signal a crucial warning" as "only absolute force" would have an effect on a US leader "bereft of reason."
Trump fired back with gusto.
"If North Korea does anything in terms of even thinking about an attack on anybody we love or we represent or our allies or us, they can be very, very nervous," he told reporters, with Vice President Mike Pence at his side.
"And they should be... because things will happen to them like they never thought possible."
'Better get their act together'
Relations between Washington and Pyongyang have been tense for months, in the wake of the North's repeated missile tests, including two successful intercontinental ballistic missile (ICBM) test launches in July.
Those launches put the US mainland in range. Some experts believe the second missile could potentially reach New York.
The escalating saber-rattling took an unexpected turn Tuesday when Trump seemed to borrow from the North's arsenal of rhetoric and said it faced "fire and fury like the world has never seen" if it continued to threaten the US.
That prompted a defiant Pyongyang to threaten the missile attack on Guam.
The war of words has set off diplomatic alarm bells, and raised fears of a miscalculation that could lead to catastrophic consequences on the Korean peninsula and beyond.
The region is facing "a mini Cuban missile crisis," John Delury, a professor at Seoul's Yonsei University, told AFP.
And speaking at an event in California, Mattis appeared intent on dialing back the tension, explaining his mission and responsibility was to have military options ready "should they be needed" but stressing the US effort is currently focused on diplomacy.
"The tragedy of war is well-enough known it doesn't need another characterization beyond the fact that it would be catastrophic," he added.
Mattis did not elaborate on the diplomatic results he referenced, but the UN Security Council at the weekend passed a new set of sanctions against Pyongyang over its weapons program, including bans on the export of coal, iron and iron ore, lead and lead ore as well as fish and seafood.
The new punishment could cost North Korea $1 billion a year, and even China voted for the US-drafted proposal.
Trump dismissed the notion that his administration was delivering mixed messages and said Washington remained open to negotiations.
But he once again suggested that he expected China to "do a lot more" to bring North Korea into line.
"I will tell you this, North Korea better get their act together or they're going to be in trouble like few nations have ever been in trouble in this world, okay?" he added.
In North Korea, General Kim Rak-Gyom, the commander of the North's missile forces, dismissed Trump's "fire and fury" remarks as "a load of nonsense," according to Pyongyang's official Korean Central News Agency.
"Sound dialogue is not possible with such a guy bereft of reason," he added.
The military was expected to finalize its Guam plan by mid-August and submit it to Kim for consideration,he said.
The unusually precise statement said the four missiles would be launched simultaneously and overfly the Japanese prefectures of Shimane, Hiroshima and Kochi.
They would have a flight time of 17 minutes 45 seconds, travel 3,356.7 kilometers (around 2,086 miles) and come down 30 to 40 kilometers away from Guam, it said -- just outside US territorial waters.
Japan, which has in the past warned it would shoot down any North Korean missiles that threaten its territory, responded that it could "never tolerate" provocations from the reclusive state.
Analysts said a North Korean launch towards Guam would put the US in a dilemma: if it did not try to intercept the missiles, its credibility would be damaged and the North would feel emboldened to carry out a full-range ICBM test.
But if an intercept were attempted and failed in any way, it would undermine the effectiveness of the United States' ballistic missile defense system.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)