The chairman of South Korea's main opposition party resigned on Sunday in an effort to pave the way for the nation's liberals to select a single candidate ahead of the December presidential poll.

Lee Hae-Chan quit along with several other top officials of the Democratic United Party (DUP) following calls for internal party reforms seen as necessary for the nation's two liberal presidential candidates to merge their campaigns.

Moon Jae-In, the party's own candidate, and Ahn Cheol-Soo, a popular software mogul running as an independent, have been in talks to select a single candidate to challenge the conservative front-runner Park Geun-Hye.

The talks were however suspended last week after the Ahn camp accused Moon of lacking sincerity for promised efforts for political reforms, including the removal of top DUP officials he says are responsible for past policy failures.

Moon later apologised to Ahn and urged him to return to the negotiation. The two earlier promised to announce a decision before November 25 -- the final deadline for candidate registration before the December 19 election.

If both men end up running, they will split the liberal vote and effectively hand the presidency to Park from the ruling New Frontier Party who has a lock on the sizeable conservative bloc.

"We are stepping down because selecting a single candidate for the eventual administrative change is such an important and urgent goal," Lee told reporters.

"We do not want to become an obstacle that further delays the merger talks...the election is only a month away. Time is running out," Lee said, adding Moon would serve as the temporary party chief.

Ahn in response promised to meet Moon again "as soon as possible".

While polls suggest Park would easily win in the event of a three-horse race, they put her neck and neck in a face-off with either Moon or Ahn.

Moon's camp had been especially vocal on the need for a unified candidacy. Ahn's side has been more cautious, insisting on a commitment from Moon's party to political reform.

Ahn has virtually no political experience but is enormously popular with young liberal voters, who see him as untainted by corruption or by political or commercial abuse of power.

Moon's supporters argue that their man would make the better candidate as he has the party base and political experience necessary for the president in dealing with parliament.