US President Joe Biden and his South Korean counterpart Yoon Suk Yeol warned North Korea it would face a nuclear response and the "end" of the leadership there if Pyongyang uses its own arsenal.
Speaking at the White House after Oval Office talks during only the second state visit so far in the Biden presidency, the two leaders said the US security shield for South Korea was being strengthened in the face of the nuclear-armed North's aggressive missile tests.
And they made clear that if the isolated, communist dictatorship in North Korea attacks the South or the United States, the response will be devastating.
"A nuclear attack by North Korea against the US or its allies... will result in the end of whatever regime were to take such an action," Biden told reporters at a joint press conference with Yoon.
Yoon said his priority was to secure peace through "superiority of overwhelming forces and not a false peace based on the goodwill of the other side."
"In the event of a North Korean nuclear attack," he said, Washington and Seoul have agreed to "respond swiftly, overwhelmingly and decisively using the full force of the alliance including US nuclear weapons."
A military honour guard and hundreds of guests massed outside the White House where Yoon and his wife, Kim Keon Hee, arrived for a day of pomp and ceremony.
They rounded off the day with a lavish state dinner where Hollywood star Angelina Jolie was among the guests joining the Korean first couple, Biden and First Lady Jill Biden.
In a toast, Biden recalled the sacrifice by American soldiers to help fight the communist north during the 1950-1953 Korean War and said their countries were bound by "a belief in democracy, liberty, security and above all a mutual belief in freedom."
Yoon and Biden issued what was titled the Washington Declaration, bolstering the US nuclear umbrella over South Korea, which is increasingly nervous about the sabre-rattling in the north.
"President Biden has reaffirmed his ironclad commitment to extended deterrence towards the Republic of Korea," Yoon said.
This will include a mechanism for the two countries to share information and consult in event of a North Korean attack, even if US commanders will still retain full control on the nuclear weapons.
It will also see more integration of South Korea's conventional military with US nuclear forces.
A senior US official, speaking on condition of anonymity, described the new arrangement as an echo of moves last witnessed when Washington oversaw the defence of Europe against the Soviet Union.
"The US has not taken these steps, really, since the height of the Cold War with our very closest handful of allies in Europe. And we are seeking to ensure that by undertaking these new procedures, these new steps, that our commitment to extended deterrence is unquestionable," the official said.
US officials, speaking on condition of anonymity, stressed that there are no plans to station nuclear weapons in South Korea – a difference from the Cold War, when US strategic weapons were deployed to Europe.
In addition, Seoul reiterated its pledge in the declaration not to seek its own nuclear arsenal.
The US official said initial steps would include "regular deployment of strategic assets, including a US nuclear ballistic submarine visit to South Korea, which has not happened since the early 1980s."
In addition to submarines, there will be a "regular cadence" of other major platforms, "including bombers or aircraft carriers," the official said, emphasising however that there will be "no basing of those assets and certainly not nuclear weapons."
A US official said that steps are being taken in advance to defuse potential tensions with Beijing over the tougher military posture.
"We are briefing the Chinese in advance and laying out very clearly our rationale for why we are taking these steps," the official said, adding that the Biden administration is "disappointed that China has been unprepared to use its influence" on North Korea.
Yoon will address a joint session of Congress on Thursday and have lunch with Vice-President Kamala Harris and Secretary of State Antony Blinken.
On Friday, he will visit MIT and Harvard University in Boston, before returning home on Saturday.
Yoon and Biden on Tuesday visited the Korean War Memorial, which features life-sized steel statues of US soldiers.
Yoon also laid a wreath at the Tomb of the Unknown Soldier in Arlington National Cemetery and joined Harris for a tour of a Nasa facility near Washington.