Harris also cautioned that the missile launch indicated a major advancement in the North's technological capabilities and called for further coordination among Japan, South Korea and the USA to deal with the increasing North Korean threat.
Haley also criticised Moscow, after Russian President Vladimir Putin responded to North Korea's latest missile launch by saying that while his country opposed Pyongyang's nuclear programme, it was unacceptable for countries to intimidate North Korea.
North Korea conducted two nuclear tests past year, which are likely to have improved its know-how in making nuclear weapons small enough to fit on long-range missiles.
The South Korean president declared support for a two-track policy - sanctions and dialogue - to denuclearize the North. Moon also noted the seemingly significant advancement in the North's nuclear and missile capabilities. All three countries have helped minimize economic and financial sanctions imposed on North Korea.
"We are willing to talk, but not until we see a total stop of the nuclear process and of any tests there", Haley said.
The one-month shutdown of the aged power plants will have little impact on the country's overall power supplies because they could easily be replaced by more costly but less polluting LNG power plants largely run by private companies, the presidential office said. Law of the Sea Hague Tribunal that declared China's territorial claims over essentially the entire South China Sea as illegal.
Pyongyang announced that the Hwasong-12 intermediate-range ballistic missile flew 490 miles, but at an apogee of 1,300 miles. But he has said that North Korea must change its attitude of insisting on pressing ahead with its arms development before dialogue is possible.More news: Probe has Trump adviser under scrutiny
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In this undated image made from video, members of CD REV chant about THAAD, the U.S. Army's missile defense system formally known as Terminal High Altitude Area Defense.
North Korea has justified its latest long-, medium-range ballistic missile test-firing as "routine work to raise nuclear capability for self-defence", rejecting the UN Security Council's condemnation.
President Moon Jae-in shakes hands with Matthew Pottinger, senior director for East Asia at the White House's National Security Council at the Blue House in Seoul, May 16.
US President Donald Trump's administration has called for a halt to Pyongyang's provocations and warned that the "era of strategic patience" with North Korea is over.
Yang also told the delegation that China "will always closely discuss Korean Peninsula issues with South Korea without exceptions and will continue to cooperate with South Korea for the peace and stability of the peninsula". "Calling the complete dismantlement of Pyongyang's nuclear programs an ultimate goal, Moon and Trump both believe that all possible means including sanctions and talks should be used".
Speaking to the Associated Press about what would happen in the event of a nuclear strike from the North, scientist David Wright, of the UCS Global Security Program, and rocket analyst Markus Schiller, of ST Analytics in Germany, described how the drama would unfold.
Moon, a liberal who was elected last week after a decade of conservative leadership, has taken a relatively diplomatic tone with North Korea, with whom the South remains technically at war following the armistice signed to end the 1950-1953 conflict.