President Donald Trump on Tuesday warned North Korea he was prepared to use the full range of US military power to stop any attack but urged Pyongyang to "make a deal" to end the nuclear standoff.
Speaking on North Korea's doorstep during a visit to Seoul, Mr Trump's appeal for talks was more conciliatory than ever before.
The president said that while "we hope to God" not to have to resort to the use of full US 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," Mr Trump said after talks with South Korean President Moon Jae-in, who has supported diplomacy with Pyongyang.
But at times taking a more measured, less confrontational tone, Mr Trump also urged North Korea to "do the right thing" and added that: "I do see some movement," though he declined to elaborate.
"It really makes sense for North Korea to come to the table and make a deal," Mr Trump said at a joint news conference with Mr Moon.
Despite Mr Trump's renewed threats against North Korea, it was 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.
North Korean leader Kim Jong-un has made clear, however, that he has little interest in negotiations, at least until he has developed a nuclear-tipped missile capable of hitting the US mainland.
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 24-hour visit that could aggravate tension with North Korea.
He then flew by helicopter to Camp Humphreys, the largest US military base in the country, and met US and South Korean troops, along with Mr Moon.
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The White House billed Mr Trump's trip as intended to demonstrate US resolve over a hardline approach to the North Korean nuclear and missile threats.
But many in the region had expressed fear that any further bellicose rhetoric by Mr Trump toward Pyongyang could increase the potential for a devastating military conflict.
Mr Trump praised Mr Moon for "great cooperation" despite differences in the past over how to confront North Korea and over a trade pact between the United States and South Korea.
The leaders said they had agreed to renegotiate the trade agreement in a timely fashion.
In formal talks after an elaborate welcoming ceremony outside the presidential Blue House in Seoul, Mr Moon told Mr Trump he hoped his visit would relieve some of South Koreans' anxiety over North Korea.
Pyongyang’s recent nuclear and missile tests in defiance of UN resolutions and an exchange of insults between Mr Trump and Mr Kim have raised the stakes in the most critical international challenge of Mr Trump’s presidency.
At the news conference, Mr Trump said Pyongyang must understand the "unparalleled strength" that Washington had at its disposal.
He cited three US 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.
Mr Trump has rattled some US allies with his vow to “totally destroy” North Korea if it threatens the United States and by deriding Kim as a “Rocket Man on a suicide mission”.
Mr Kim responded by calling Trump a “mentally deranged US dotard.”
Mr Trump's senior aides privately have since urged him to avoid "personalising" the conflict any further.
On the second leg of his five-nation trip, Mr Trump toured the sprawling Camp Humphreys garrison, which lies about 100 kilometres from the border with North Korea, and met commanders and troops.
Mr Trump wrapped up his first day with a dinner hosted at the Blue House, dining on grilled sole, beef ribs and chocolate cake while being serenaded by a K-pop singer with an orchestra in the background.
North Korea has not conducted a missile test for 53 days, the longest such lull in testing this year. North Korean state media has not commented on Mr Trump's arrival in the South.
South Korea’s spy agency said last week that North Korea may be preparing another missile test, raising speculation that such a launch could be timed for Trump’s trip to the region.
US officials have said privately that intercepting a test missile is among options under consideration, though there is disagreement within the administration about the risks.