South Korean President Moon Jae-in says the confrontation over North Korea's nuclear program must "absolutely be solved peacefully" and the U.S. would need South Korea's approval to take military action.
Moon, a liberal, said in a nationally televised address on Tuesday local time his government "will put everything on the line to prevent another war in the Korean Peninsula."
South Korea favors diplomatic engagement with North Korea. In an early move as president, Moon suspended the deployment of a U.S. missile defense system known as THAAD.
But, Moon later changed course after North Korea's escalating missile tests and the U.S. and South Korea are now moving forward with deploying THAAD on the Korean Peninsula.
President Trump last week threatened to impose "fire and fury" on North Korea's regime.
On Monday, Defense Secretary Jim Mattis described any potential military action as defensive in nature.
He warned: "If they fire at the United States, it could escalate into war very quickly."