Ban and Moon currently leads opinion polls on next year's election, with Lee taking up the third spot.

"One of the key criteria (in terms of candidates' qualification) is their achievement from previous public posts, and whether they abused the posts for personal interests. In that sense, Ban is different from me," Lee said during an interview with Yonhap News Agency.

Lee said Ban took advantage of his post by sending a letter to Kim Jong-pil, a former prime minister and an iconic conservative figure, through a diplomatic bag.

"We need someone who holds the willingness and courage to fiercely fight with the privileged class with corruption, and in that sense I am better (compared to Moon)," Lee also said.

Lee, however, said Moon, along with Seoul Mayor Park Won-soon, will join forces regardless of who wins.

The mayor said it is "unimaginable" to cooperate with the planned New Conservative Party for Reform, which departed from the ruling Saenuri Party, adding it is also responsible for the influence-peddling scandal of President Park Geun-hye which rocked the country for past months.

Lee also vowed to bring major changes to the policies implemented by Park.

Concerning the Seoul-Tokyo deal reached last year on the sexual enslavement of Korean women during Japan's 1910-45 colonial rule, Lee said the agreement should be reviewed from the scratch, adding it holds no legal significance.

South Korean and Japan reached a deal in December 2015, where Tokyo agreed to provide 1 billion yen ($9.61 million) for the creation of a foundation aimed at supporting the victims, euphemistically called comfort women.

Lee said South Korea should also withdraw the decision to deploy an advanced US missile defense system in the country, adding Seoul could become a victim of military conflict with Washington and Beijing. (Yonhap)