Speaking on North Korea's doorstep during a visit to Seoul, Trump said that while "we hope to God" not to have to use all of the United States' military might, he was ready to do whatever was necessary to prevent the "North Korean dictator" from threatening millions of lives.
"We cannot allow North Korea to threaten all that we have built," Trump said after talks with South Korean President Moon Jae-in, who has supported diplomatic outreach to Pyongyang.
But at times taking a more measured, less confrontational tone, Trump also urged North Korea to "do the right thing" and said: "I do see some movement", though he declined to elaborate.
"It really makes sense for North Koreas to come to the table and make a deal," Trump told reporters at a joint news conference with Moon.
Despite Trump's renewed threats against North Korea, it was still a far cry from the more strident approach he has pursued in recent months, including his previous dismissal of any diplomatic efforts with Pyongyang as a waste of time.
Landing earlier at Osan Air Base outside Seoul, the president and First Lady Melania Trump stepped down from Air Force One onto a red carpet as he began a visit that could aggravate tension with North Korea.
He then flew by helicopter to Camp Humphreys, the largest U.S. military base in the country, and met U.S. and South Korean troops, along with Moon.
The White House billed Trump's trip as intended to demonstrate U.S. resolve over a hardline approach to the North Korean nuclear and missile threats.
But many in the region have expressed fear that his bellicose rhetoric toward Pyongyang could increase the potential for a devastating military conflict on the Korean peninsula.
Meeting with military commanders about the North Korea issue, Trump told reporters: "Ultimately it will all work out, it always works out, it has to work out." He did not elaborate.
Trump praised president Moon, hailing him for "great cooperation" despite differences over how to confront North Korea and over a trade pact between the United States and South Korea.
In formal talks after an elaborate welcoming ceremony outside the presidential Blue House in Seoul, Moon told Trump he hoped his visit would relieve some of South Koreans' anxiety over North Korea and serve as a "turning point in resolving the North Korean nuclear issue".
Pyongyang's recent nuclear and missile tests in defiance of U.N. resolutions and an exchange of insults between Trump and North Korean leader Kim Jong Un have raised the stakes in the most critical international challenge of Trump's presidency.
At the news conference, Trump said Pyongyang must understand the "unparalleled strength" that Washington had at its disposal.
He cited three U.S. aircraft carrier strike groups that are converging on the Western Pacific for exercises as well as a nuclear submarine he said was also in position.
On the second leg of his five-nation trip, Trump toured the sprawling Camp Humphreys garrison, which lies about 100 km (60 miles) from the border with reclusive North Korea.
Trump was greeted with applause and a few cheers as he and Moon entered the mess hall at lunch hour.
Trump is seeking to ratchet up pressure on Pyongyang following his visit to Tokyo, where he declared that Japan would shoot North Korean missiles "out of the sky" if it bought the U.S. weaponry needed to do so, suggesting the Japanese government take a stance it has avoided until now.
(Additional reporting by Soyoung Kim, James Pearson, Josh Smith and Hyonhee Shin in Seoul, Phil Stewart and Idrees Ali in Washington; writing by Matt Spetalnick; Editing by Nick Macfie and Clarence Fernandez)
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)