SEOUL - South Korea's former prime minister Han Duck-soo said on Tuesday he saw no alternative other than teaming up with the presidential candidate of the ruling conservative People Power Party to contest the country's snap election on June 3.

Han emphasized the need to run on a unity ticket when asked whether he would run separately from former labor minister Kim Moon-soo, who was picked by the PPP as its candidate earlier this month.

"I've never thought that unification would fail because that would be a great betrayal of the people," he said at a debate hosted by the Kwanhun Club, a grouping of senior journalists.

Lee Jae-myung, the candidate of the main opposition Democratic Party, remains a clear frontrunner in the election, piling pressure on conservatives to put forward one candidate to avoid splitting the vote.

Lee, the liberal candidate, had nearly 50 percent support, according to a survey by the JoongAng Ilbo daily published on Tuesday.

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The survey put Kim on 33 percent if he became the candidate on a unity ticket, while Han was on 36 percent if he ran as the candidate in a three-way contest with Lee and Lee Jun-seok, the minor opposition New Reform Party's candidate.

Kim criticized his own party in a Facebook post on Tuesday, saying he was not included in the process of running a joint candidacy.

Former prime minister Han also said he felt confident that he was best placed to resolve trade issues as the next president and vowed to find a "win-win solution" in talks with the United States.

"I think I'm the one who's been dealing with trade issues between South Korea and the US for the longest time, who can do the best job," he said.

South Korean and US officials held the first round of trade talks in Washington last month as Seoul pushes to cut a deal that could curb the impact of US tariffs on vital sectors like the auto industry.

Han oversaw the talks while serving as acting president following the impeachment of then president Yoon Suk-yeol over his short-lived imposition of martial law.

Han stepped down as acting leader last week and announced his entry into the presidential race, hoping to leverage his higher profile after a spell in the leadership role.

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Yoon was ousted by the Constitutional Court last month, prompting the snap presidential election.