South Korea will not develop atomic weapons of its own despite the threat from the nuclear-armed North, President Moon Jae-In declared on Wednesday.
"A push by North Korea to become a nuclear state cannot be accepted or tolerated," Moon said in an address to parliament. "We also will not develop or own nuclear" arms.
In recent months Pyongyang has carried out its sixth nuclear test -- its most powerful by far -- and launched missiles apparently capable of reaching much of the US mainland, raising concerns in Seoul about its security alliance with Washington.
South Korean media and opposition politicians have called for US tactical nuclear weapons, which were withdrawn from the peninsula in the 1990s, to be returned.
Some have suggested that if Washington does not agree -- Defense Secretary Jim Mattis expressed doubts about the concept in a visit at the weekend -- Seoul should develop a nuclear capability of its own, in order to ensure what they dub a "balance of terror" on the peninsula.
But Moon said in his address that Seoul's approach would be "based on the joint declaration to denuclearise the Korean peninsula declared by both Koreas" in 1992.
Then the two Koreas agreed not to develop nuclear arsenal on the flashpoint peninsula, and two years later the North forged an aid-for-denuclearisation deal with the US.
The 1994 deal fell apart in 2002 when the North walked out and resumed its atomic weapons programme after Washington raised suspicions Pyongyang was secretly pursuing nuclear arms.
Pyongyang carried out its first atomic test in 2006, and has made significant progress in its weapons technology under current leader Kim Jong-Un, who has overseen four atomic blasts and numerous missile tests since inheriting power in 2011.
The North hails its nuclear arsenal as a "treasured sword" to protect itself from potential invasion by its "imperialist enemy" the US, but has threatened to bracket the US Pacific island of Guam with missiles.
Kim and Donald Trump have also traded personal insults in recent months, sparking concerns of a conflict on the peninsula where the 1950-53 Korean War left millions dead.
Tensions escalated further as Trump warned of "fire and fury" against the North and a "calm before the storm".
But Moon insisted there could be no US military action without Seoul's agreement, saying Koreans had to "determine the fate of our nation ourselves".
"There should be no military action on the peninsula without our prior consent," he said.
"We will not repeat the tragic history like colonialisation and division during which the fate of our nation was determined regardless of our will," he added.
Japan colonised the peninsula from 1910 to 1945, and after Tokyo's surrender ended the Second World War it was divided into separate zones of occupation by Russia and the US.
Even some Trump advisers say US military options are limited when any armed conflict on the peninsula is expected to cause massive casualties.
The South's capital Seoul is home to 10 million people and only about 50 kilometres (30 miles) from the border, within range of Pyongyang's artillery.
One study by the Nautilus think-tank in California estimated around 65,000 civilians would die in Seoul alone on the first day of a conventional North Korean attack.
Trump is scheduled to visit Japan, South Korea, China, Vietnam and the Philippines during his first Asia trip this month, with all eyes on his message to the North and Kim.
How to curb the North's threats is expected to top the agenda when Trump visits the South -- a key Asian ally of Washington's, which hosts 28,500 US troops -- for a summit with Moon on November 7.