"In face of the impeachment and arrest of an incumbent president, our people opened the path toward the future for the Republic of Korea", said Moon, referring to South Korea's formal name. Moon's aides say he will commute from his home for the first few days as workers finish furnishing the Blue House residential space for him.
The THAAD deployment was agreed by Moon's predecessor, conservative leader Park Guen-hye.
South Korea's president-elect Moon Jae-in gestures to supporters at Gwanghwamun Square in Seoul, South Korea, May 9, 2017.
Echoing the United States' line, Moon also suggested that China - the North's sole major ally and economic lifeline - should do more to tame Pyongyang, saying "solving the THAAD problem would be easier if there was no more provocation by the North".
Newly elected South Korean President Moon Jae-in chose to send special envoys to China, Russia, Japan and the United States, the presidential office said Thursday. Moon also has questioned the deployment of a USA missile defense system the last government rushed into service.
North Korea is likely to welcome Moon's election but its state media made no mention of his victory on Wednesday.
Chinese President Xi Jinping said he "always attaches great importance to the relationship between China and South Korea", and that he was "willing to diligently work with" Moon to ensure both countries benefit, Xinhua reported.
Keeping Washington, Seoul, and Tokyo all pulling in the same direction to address common threats will require deft diplomacy as the new South Korean president and President Donald Trump fill their administrations and formulate policies. With turnout at 77 percent, the highest level in two decades, Moon can claim a genuine mandate.
"It remains a concern that the left of centre, left-wing party in South Korea is going to do well", the official told Reuters, asking not to be identified.
Beijing will be hoping for concessions from Moon to get relations back on track after months of Chinese fury over South Korea's decision to deploy THAAD.
Beijing also has its own issues with Seoul.
He also advocates resumption of some inter-Korean projects shuttered under the conservative governments of the last 10 years, including the Kaesong joint industrial zone, where South Korean firms employed Northern workers.
South Korea has upset China, its biggest trading partner, by agreeing to the deployment of a US missile defense system on its territory.
The leaders of China, Japan and other countries sent their congratulations. Moon opposes a 2015 agreement signed by Park that was heralded as a final settlement for Korean women who were among many sexually enslaved in Japanese military brothels before and during World War II.
Trump warned late last month that he intends to renegotiate or terminate the deal, which came into effect in 2012.
The South Korean official told Yonhap that possibility for an encounter between the two Korean participants may not be ruled out. On a visit to Seoul during the presidential campaign, Tillerson insisted that the North would first have to give up its entire arsenal before talks began - even though dismantling that arsenal is the ultimate goal of those negotiations.
Moon's approach differs from Washington's stance towards Pyongyang.
"South Korea has tremendous influence in the worldwide community on this issue, and that in itself is a challenge for President Trump", Straub said, noting that Kim Dae-jung and Roh both badmouthed president George W. Bush's approach at that time.
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