Main opposition presidential candidate Yoon Suk-yeol has said he will not hold negotiations with minor candidate Ahn Cheol-soo for a merger of their candidacies while leaving open the possibility of a merger through a surprise deal.

Yoon of the conservative People Power Party made the comments in an interview with the Joongang Ilbo newspaper published Wednesday, emphasizing his aversion to long, drawn-out negotiations over details.

"If we trust each other and agree on the goal of regime change, then it can be over in less than 10 minutes over a cup of coffee," he said.

"If they ask me to carry out negotiations through something like a merger preparatory committee, then I won't do it."

A political merger has emerged as the single largest variable in the presidential race with exactly four weeks to go until the March 9 election.

Ahn of the centrist People's Party has insisted he has no plans to merge candidacies and will complete the race on his own.

But pressure has been building to form a merger and increase the opposition's chances in the election amid neck-and-neck poll ratings of under 40 percent for both Yoon and ruling party candidate Lee Jae-myung of the liberal Democratic Party (DP).

"If we were to do (a merger), we would have to do it suddenly," Yoon said, refusing to discuss specifics, such as whether it could take place before the Feb. 14 deadline for registering candidacies with the National Election Commission, the election watchdog.

Yoon reaffirmed his belief in the need to deploy additional units of the US THAAD anti-missile system in South Korea but not through US Forces Korea.

"I'm saying we should pay 1.5 trillion won ($1.25 billion) to buy it ourselves," he said. "My understanding is that during the THAAD deployment (in South Korea) in 2016, China was most concerned about THAAD's X-band radar searching its inland and affecting its military strength vis-a-vis the United States."

Yoon also said he believes there will be a reinvestigation into allegations Lee masterminded a massively corrupt development project carried out in Seongnam, south of Seoul, when the DP candidate was the city's mayor.

"If the investigation is done by prosecutors who are in their right minds, would they say Yoo Dong-gyu was responsible for all of it?" he said, referring to a former key aide to Lee.

"The person with authority and able to do the decision making was the mayor." (Yonhap)