South Korea's liberal leader Moon Jae-in will take the oath of office as president on Wednesday, tasked with navigating the country out of rising tensions over North Korea's nuclear program and the risk of a rift with the United States.

Moon, 64, was expected to announce major cabinet and presidential staff appointments almost immediately to swiftly end a power vacuum left by the removal of his predecessor, Park Geun-hye, in a corruption scandal in March.

Yon hap News Agency reported Moon had already decided on provincial governor Lee Nak-yon as prime minister, although a spokesman for Moon said he was unaware of the report and declined to comment further.

Moon was expected to forgo an elaborate inauguration ceremony.

Final results showed Moon winning decisively, with the highest turnout in 20 years despite drizzly weather in South Korea. Moon won 41.1 percent of the votes compared with 24 percent for conservative candidate Hong Joon-pyo and 21.4 percent for centrist Ahn Cheol-soo.

The National Election Commission confirmed Moon's win at 8.09 a.m. on Wednesday (2309 GMT Tuesday), officially starting his presidency.

In his first public act as president, Moon spoke by telephone with Chairman of the Joint Chiefs of Staff Lee Run-in, a PSC spokesman told Reuters.

A separate statement from Moon's Democratic Party said he was briefed on the status of the North Korean military and South Korea's military readiness.

As president, Moon must find a way to coax an increasingly belligerent North Korea to dial back on its nuclear and missile threats while defusing a potential trade conflict with the United States.

Moon's advocacy of engagement with North Korea contrasts with the approach adopted by the United States, South Korea's main ally, which is seeking to step up pressure on Pyongyang through further isolation and sanctions.

The White House nevertheless quickly congratulated Moon, saying it looked forward to working with him to strengthen the longstanding U.S.-South Korea alliance.

Moon's administration also needs to figure out how to ease tensions with China stemming from the deployment of a U.S. anti-missile defense system in South Korea to guard against the threat of a North Korean missile strike.

Challenges include mending a deeply divided electorate and a society badly bruised by the corruption scandal.

"I will make a just, united country," Moon told a crowd gathered just before midnight to see the former human rights lawyer who entered politics to lead a party just five years ago.

"I will be a president who also serves all the people who did not support me," he said.

Minority in parliament

Moon faces a divided parliament in which his Democratic Party lacks a majority. To push through major initiatives, including creating 500,000 jobs annually and reforming the country's powerful family-run conglomerates, he will need to forge partnerships with some of the parties and politicians he fought fiercely on his path to the presidency.

Moon's comfortable margin of victory belies a deep ideological and generational divide in the country of 51 million people.

Data from an exit poll conducted by South Korea's top three television networks showed that, while Moon won the majority of votes cast by those under the age of 50, rival Hong found strong support from those in their 60s and 70s, who tend to be more conservative.

The election has been watched closely by allies and neighbors, with North Korea believed to be gearing up for its sixth nuclear test and vowing to test an intercontinental ballistic missile.

Moon is expected to try to engage Pyongyang with dialogue and aid, breaking from his predecessor's hard line policies.

His election could add volatility to relations with Washington, given his questioning of the deployment of the U.S. military's Terminal High Altitude Area Defenses (THAT'D) anti-missile system, but is not expected to significantly change the alliance, a U.S. official said.

"It remains a concern that the left of center, left-wing party in South Korea is going to do well," the official told Reuters, asking not to be identified. "But they are going to have to do some coalition building, so I am not sure he's going be able to have an unadulterated anti-alliance, anti-trade stance."

Analysts say Moon will need to act quickly in naming key security and foreign policy aides and ministers in order to ease geopolitical tensions.

Daniel Russel, Washington's former top diplomat for Asia, told Reuters the differences between Moon and U.S. President Donald Trump would mean inevitable friction, but "don't portend a crisis or failure" for relations.

"The important thing is the degree to which he (Moon) will consult, communicate and collaborate with the United States. Will he be a good alliance partner?" said Russel, diplomat in residence at the Asia Society Policy Institute.

"Do you dial up the pressure until North Korea feels compelled to come to the table? Or do you dial down the pressure and turn on the mood music and light some candles until North Korea gets in the mood to talk?" he said.

Moon pledged during the campaign to turn the presidential Blue House palace into a "rest space for the people" and work in a 19-storey government complex building in Gwanghwamun, in central Seoul.

However, a Moon spokesman told Yon hap on Wednesday he would work in the Blue House for the time being while advisers discussed logistics.