The South Korean leader has warned that Pyongyang completing an intercontinental ballistic missile (ICBM) and weaponizing it with miniaturized nuclear weapon is "nearing" a red line.
Mr Trump had declared the U.S. military "locked and loaded" and said he was ready to unleash "fire and fury" if North Korea continues to threaten America.
However, next week will see the start of US-South Korean military exercises which have angered the North in the past.
His comments follow a spike in animosity generated by North Korea's warning that it might send missiles into waters near the U.S. territory of Guam, and Mr Trump's warlike language.
"Also, even if the United States takes military action outside the peninsula, I am confident it will sufficiently consult with South Korea in advance if such action may increase tension between the South and the North", he added.
The chairman of the U.S. Joint Chiefs of Staff, Marine Corps Gen. Joseph Dunford, on Monday met with senior South Korean military and political officials and the local media, seeking to ease anxiety while showing his willingness to back Trump's warnings if need be.
"We hope for a peaceful resolution but we needed to seriously have a conversation about what might happen if there was military action", Dunford said.
US: War would be 'horrific' but North Korea nukes 'unimaginable'
Mr Trump wrote: "The alternative would have been both catastrophic and unacceptable!"
The United States should move some of its nuclear weapons arsenal to South Korea in a bid to deter the increasingly threatening North, the main opposition party in Seoul said earlier today.
Washington has 28,500 troops stationed in the country to protect it from the North. He said no USA military action on the Korean Peninsula could be taken without Seoul's consent. This would involve negotiating with the United States to redeploy some of its own nuclear weapons to the country.
Though he suspended the deployment of a US missile defense system opposed by China, he reversed that decision last month after North Korea tested two intercontinental ballistic missiles.
Pyongyang carried out its first successful tests of an intercontinental ballistic missile (ICBM) last month, apparently bringing much of the U.S. mainland within its reach.
Washington and Trump had agreed that "no matter what option they take about North Korea, all decisions will be made after consulting with and getting agreement with the Republic of Korea".
"This is the same as China's solution to the North Korea nuclear issue", Hua said. The liquid-fuel missile is created to be fired from road mobile launchers and has been described by North Korea as built for attacking Alaska and Hawaii.
Ronaldo, Messi lead the pack
Rafael Nadal added: "Shattered by what has happened in Barcelona!" Espanyol, the city of Barcelona's other top-flight team, tweeted: "The most handsome city in the world, our city".