While Moon has proposed for greater engagement with the isolated North, Washington wants to increase pressure on North Korea through further sanctions and isolation.
(Kim Hong-ji/Pool Photo via AP).
The Thaad system "is operational and has the ability to intercept North Korean missiles and defend the Republic of Korea", a spokesman for US Forces Korea confirmed on May 2.
Moon's first schedule as president was expected to be a morning visit to the National Cemetery in the central city of Daejeon, where the country's independence fighters and war heroes are buried.
Moon has criticized that haste and argued that the missile's deployment should have been contingent on a vote in the country's National Assembly.
South Korea's new President open to visiting the North over its nuclear progamme, missiles.
Moon also spoke with China's President Xi Jinping and US President Donald Trump about North Korea tensions.
His election could complicate the deployment of a U.S. Terminal High Altitude Area Defence (THAAD) anti-missile system, which the former Seoul government and the U.S. military agreed a year ago to deploy in South Korea as defence against North Korean missiles.
China's president has set out his opposition to the deployment of a USA missile system, in his first discussion with the new South Korean leader.
The new South Korean president faces the daunting task of finding a way to restore relations without upsetting global partners such as the US, given Pyongyang's self-description as a nuclear weapon state.
"I still believe Moon Jae-in faces major difficulties in reversing the THAAD deployment or even making any kind of modifications", Zhao says. "In order to bring about peace and security on the Korean Peninsula I will do everything that I can", he said.
At the same time, tensions between Pyongyang and Beijing have also risen in recent weeks as China works with the U.S.to rein in North Korea. "If the conditions shape up, I will go to Pyongyang", Mr Moon said.
At home, Mr Moon begins his term facing multiple challenges, including the aftermath of the huge corruption scandal that saw his conservative predecessor Park Geun Hye impeached.
Moon's election signalled the return of the liberals to the centre stage of South Korean diplomacy.
Veteran liberal politician Lee Nak-yon was nominated to serve as prime minister.
Moon also had a telephone call with Japanese Prime Minister Shinzo Abe, Japanese news agency Jiji reported.
"Politics were turbulent (in the past several months), but our people showed greatness", Moon said.
During Thursday's call, Xi reaffirmed China's opposition to the THAAD deployment, Moon's office said.
In short, Moon talks the talk, and today he'll become the leader of South Korea. "We must put the days of fierce competition behind and hold hands marching forward".
He has also been critical of the two previous conservative administrations for failing to stop North Korea's weapons development. "Our two countries have extensive cooperation on political, economic, cultural and people-to-people exchanges in the bilateral, regional and multilateral spheres", he added.