Expectations that he would chart a different course on North Korea, as well as the controversial US anti-missile shield THAAD, were immediately confirmed in his inauguration speech.

A former human rights lawyer and son of North Korean refugees, Mr Moon, 64, was sworn in within hours of an electoral victory that saw him win 41.1 per cent of the vote, ending almost a decade of conservative rule.

"If necessary, I will fly straight to Washington. I will go to Beijing and Tokyo too, and I will go to Pyongyang if conditions allow it," he said.

"I will do everything I can for peace on the Korean Peninsula. On the other hand, I will negotiate seriously with the US and China to resolve the THAAD issue."

US President Donald Trump held a 30 minute phone conversation with Mr Moon on Wednesday evening, in which he told his US counterpart the South Korea-US alliance "is more important than at any other time" because of security uncertainties on the Korean Peninsula.

Robert Kelly, political science professor at Pusan National University, said it was notable that Mr Moon had stated his willingness to go to Pyongyang on day one of his presidency.

"He won't go along with Trump. He would like to talk to North Korea and he wants to go to Pyongyang," he told Fairfax Media.

In interviews ahead of the election, Mr Moon said the alliance with the US would remain a foundation of South Korea's security, but he was dissatisfied that South Korea had taken the "back seat" to the US and China over how to resolve the North Korean nuclear issue.

Mr Moon had also criticised the South Korean government for allowing the installation of the THAAD anti-missile system in the weeks before the election. China claims the radar can spy on Chinese military, and has inflicted unofficial trade sanctions against South Korean retailers and entertainers in reprisal.

Professor Kelly said the Moon government would favour engagement, including negotiating with the US and China over THAAD, but would need to tread carefully to avoid a backlash from conservatives in South Korea.

South Korea's allies, Japan and the US, would also have a lot to say if Mr Moon tried to shift too far, he said.

"He didn't win a huge mandate, but he is president. He will meet a wall of resistance."

Professor Kelly said although Mr Moon was "a man of the Left, he is not a radical".

"He wants a negotiated solution to North Korea, but he may find it is elusive. People have talked like this before."

Despite the election campaign criticism of THAAD, Professor Kelly said it was unlikely a Moon government would dismantle the radar, unless US President Donald Trump persists with his comments that South Korea should pay the $US1 billion ($1.3 billion) cost of its installation, which would give the new government an excuse.

"Moon has prevaricated on this. If he pulls out THAAD he will look like he is buckling to China and will be severely criticised by the conservative press," Professor Kelly said.

China's economic reprisals have been unpopular in South Korea.

But China was encouraged on Wednesday by Mr Moon's pledge to negotiate over THAAD.

Chinese President Xi Jinping said in a statement he welcomed Mr Moon's victory, and the two countries should "strengthen political trust, properly handle disagreements, enhance coordination and cooperation under the basis of mutual trust and respect".

"I would like to work with you to ensure the development of Sino-South Korean ties better benefits the two countries and peoples," he said.

On the People's Daily social media account, Sanjiang Huiyou, a Korea expert, wrote it was "a small country's grief" being stuck between the US and China, and Mr Moon had fewer options than he promised in his campaign.

But there were "many China hands in Moon's team so it is certain that the new administration will not be like Park's", which had neglected China and "stabbed it in the back", he wrote.

North Korea had been silent about Mr Moon during the election campaign, while heaping scorn on his conservative rivals and impeached former president Park Geun-hye.

White House press secretary Sean Spicer said the US looked forward to continuing to "strengthen the alliance" with South Korea.