Chinese leader Xi Jinping called Moon Thursday to congratulate him on his electoral victory.
He did not elaborate, but Yoon also said the two leaders agreed that all sides must work together to ease tensions over North Korea's weapons program.
Only now, the stakes are much higher.
South Korea's new liberal President Moon Jae-in was sworn in on Wednesday and vowed to immediately tackle the hard tasks of addressing North Korea's advancing nuclear ambitions and soothing tensions with the United States and China.Moon said in his first speech as president he would begin efforts to defuse security tensions on the Korean peninsula and negotiate with Washington and Beijing to ease a row over a U.S. missile defence system being deployed in the South.In a phone call congratulating Moon's election, U.S. President Donald Trump agreed with the new South Korean leader to cooperate on North Korea's nuclear issue and invited him to visit Washington, the South Korean presidential office said.Trump reaffirmed the U.S.
Regional experts have believed for months that North Korea is preparing for its sixth nuclear test.
On Thursday, the US director of National Intelligence, Dan Coats, told the Senate Intelligence Committee that North Korea posed "a very significant, potentially existential threat to the United States that has to be addressed". He endured a hard relationship with then-U.S. President George W. Bush and North Korea was a central point of contention.
Hopes of inter-Korean reconciliation have withered since Moon served as chief of staff for Roh Moo-hyun, the last South Korean leader to adopt a "sunshine" policy of diplomatic and economic outreach toward the North.
Calling Pyongyang's atomic weapons programme a "a hard problem that can be solved", Trump invited Moon to visit the United States "as early as possible", according to the statement. China contends that the system's radar ranges into its territory and threatens its own security.
He also begins his term facing multiple domestic challenges, including the aftermath of the huge corruption scandal that saw his conservative predecessor Park Geun-Hye impeached and swept him to power, but leaves the country bitterly divided.
Ahead of the swearing-in, Moon met with leading lawmakers of Liberty Korea - which advocates a hard line on the North and has repeatedly accused him of being a Pyongyang sympathizer who would "hand the whole country to the North once elected".
Given that, the prospect for improved ties between Seoul and Beijing isn't clear, with some saying China won't back down until it gets a major concession on the system's deployment and others saying China is also eager for improved ties and will settle for less as long as it can save face on the missile-defense issue. He has suggested revisiting a deal settling the historical dispute related to abuses suffered by Korean women in Japanese military brothels during World War II. Moon accepted Trump's invitation to visit at an "early date".
China is South Korea's largest trading partner, with the latter exporting up to $142 billion each year to the country.
As well as clouding efforts to rein in North Korea's nuclear ambitions, the THAAD deployment has also led to recriminations from Beijing against South Korean companies.