North Korea would be "crossing a red line" if it put a nuclear warhead on an intercontinental ballistic missile, South Korea's president says, but the US has promised to seek Seoul's approval before taking any military action.
North Korea's rapid progress in developing nuclear weapons and missiles capable of reaching the US mainland has fuelled a surge in tensions in recent days.
Pyongyang has threatened to fire missiles towards the Pacific island of Guam and President Donald Trump has warned it would face "fire and fury" if it threatened the US.
"I would consider that North Korea is crossing a red line if it launches an intercontinental ballistic missile again and weaponises it by putting a nuclear warhead on top of the missile," South Korean President Moon Jae-in said at a news conference marking his first 100 days in office.
Moon has repeatedly urged North Korea not to "cross the red line" but had not previously elaborated what that would constitute.
Trump had promised to seek negotiations and approval from South Korea before taking any options regarding North Korea, Moon also said.
US Vice-President Mike Pence told reporters in Chile on Wednesday "all options" remained on the table with regards to North Korea.
However, White House chief strategist Steve Bannon The American Prospect there was "no military solution" to North Korea's nuclear threats because of Pyongyang's massed artillery targeting the South Korean capital.
China has repeatedly urged Pyongyang to halt its weapons program and urged South Korea and the US to stop military drills in order to lower tensions.
Moon's comments came after United Nations Secretary-General Antonio Guterres said it was time to "dial down rhetoric and dial up diplomacy" and offered to help broker talks.
