After months of political turmoil, South Korea will elect a new president this week to succeed conservative Yoon Suk Yeol, who was ousted over his brief but shocking imposition of martial law.

Surveys suggested liberal Lee Jae-myung, running for the Democratic Party, is heavily favoured to win Tuesday's snap election, riding on a wave of public disappointment of Yoon’s martial law debacle in December.

The main conservative candidate, Kim Moon Soo, wants a come-from-behind victory, but observers say his refusal to directly criticize Yoon made it difficult for him to narrow the gap with Lee.

Four other politicians are running for the presidency, including Lee Jun-seok of the small conservative New Reform Party, who has categorically rejected Kim's request to field a unified candidate between them to prevent a split in conservative votes.

The winner will be sworn in as president on Wednesday without the typical two-month transition period.

The new leader faces the urgent tasks of trying to heal the deep domestic divide over Yoon's action as well as focusing on U.S. President Donald Trump's America-first policy and North Korea’s advancing nuclear programme.

When asked about Lee's plan to address the U.S. tariffs and trade policy, a senior spokesperson for the Democratic Party, Cho Seung-rae, described the South Korean-US alliance as "solid".

“The foundation of the (South) Korea-US alliance is solid and has to be advanced even further," he said.

"But advancement of the alliance is not just a matter of emphasizing the value of the alliance. Whenever administrations change in the US and South Korea, their policies can also change," he added.

People Power Party lawmaker Kim Gunn said the main point of their candidate's foreign policy was to "strengthen and upgrade" the Korea-US alliance.

"The reason this alliance is important is that for the past 70 years, the Korea-US alliance has played the role of protecting peace and stability in this region," he said.

"Due to the existence of the Korea-US alliance, countries in this region have been the biggest beneficiaries... Maintaining the strong alliance provides peace and stability to not just us but neighbouring countries," he added.

Voting officially begun on Thursday, which was the first day of a two-day early voting period.

More than 10 million people have already cast their ballots ahead of the June 3 election.

AP video by Yong-ho Kim