South Korean President Moon Jae-in says Seoul will actively try to get the nuclear negotiations between Washington and Pyongyang quickly back on track.

Moon made the comments on Monday as he led a National Security Council meeting to discuss South Korea's diplomatic strategy following the breakdown of a high-stakes summit between President Donald Trump and North Korean leader Kim Jong Un last week in Vietnam.

Moon lobbied hard for the revival of nuclear diplomacy between Washington and Pyongyang following a period of tension over the North's missile and nuclear tests and Trump's threats to bring down "fire and fury" on the North. But experts say the breakdown of the Trump-Kim meeting in Hanoi has put the credibility of Moon's role as mediator in doubt.