Yoo Seong-min of the splinter conservative Bareun Party revisited the allegation that the former presidential chief of staff consulted Pyongyang before the government abstained in a vote on UN resolution on North Korea's human rights violations in 2007, an accusation that Moon denied again.

Hong Joon-pyo of the conservative Liberty Korea Party denounced Moon for lying, citing a former foreign minister's memoirs that first sparked the controversy. Moon countered that Hong was amplifying an unverified claim.
The two-hour late night debate was hosted by public broadcaster KBS.

National security has emerged a key campaign theme for the May 9 presidential election amid growing regional tensions caused by Pyongyang's saber-rattling and Washington's show of force.

Conservatives, striving to revive their sagging campaign, focused on security issues, including the deployment of the US THAAD missile shield in South Korea, which has met with angry protests and economic retaliation from China.

As the session began, Moon raised the need for Seoul "to make it clear to China that in the event the North carries out a nuclear test, the deployment of THAAD is inevitable."

Yoo criticized Moon for inconsistency, saying he initially opposed the missile defense and then pledged to deploy it if the North conducts its sixth nuclear test.

Sim Sang-jeung of the progressive Justice Party taunted Moon's "strategic ambiguity" stance as fitting a commentator, not a political leader.

Moon dodged the blows from the minor candidates, each below 10 percent in approval ratings, and focused his offensive on closest rival Ahn Cheol-soo of the center-left People's Party.

The election is shaping up as a two-way race as Moon maintains a slight lead over Ahn, most recently by 40 percent to 37 percent in approval ratings.

Moon has called for the decision on THAAD to be left to the next administration, while Ahn is a rare political liberal officially backing the deployment.

Moon accused Ahn of limiting Seoul's options in negotiations with Washington and Beijing by taking a fixed position. Ahn's stance is out of sync with his party's official line, Moon said.

Ahn defended his position by criticizing the incumbent government for bypassing crucial consultation with China before agreeing with the United States on the deployment.

All the contenders agreed that the South Korea-US alliance and China's political and economic leverage on North Korea are key to stopping the unruly regime from conducting a new nuclear test.

At the start of debate, the five candidates were asked what diplomatic leverage the South Korean government has to block the North's provocation.

Moon said the government's first task is to discuss the issue with Washington.

"First, there's a need to closely talk and coordinate with our ally the United States, and in the process, say what we have to say to have our position clearly reflected," he said.

Hong said China's role will be important to prevent an "extreme" confrontation between North Korea and the US.

"If China can deter North Korea's provocations, I don't think the US will carry out a pre-emptive strike (against Pyongyang)," he said.

Ahn ruled out any form of war on the peninsula.

"It's important to make the US aware that we must be the main players and decide the fate of the Republic of Korea," he said. "With China, we must step up diplomatic efforts to lead it to actively take part in sanctions against North Korea."

Ahn said China's lukewarm stance was a key reason behind North Korea's continued provocations.

Yoo called for joining hands with the US to persuade China to tighten sanctions against Pyongyang by banning coal imports from the country and cutting supplies of crude oil.

He also left open the possibility of a pre-emptive strike, saying it is a self-defensive measure to be used when a North Korean nuclear attack appears imminent.

Sim stressed the importance of managing the situation together with allies and the international community to ensure that the North's strategic provocations don't spiral into a crisis.

She also argued the government should actively play a mediator role to lead China and the US to reaffirm the principle of guaranteeing peace on the peninsula.

"Based on this, I will offer a carrot and stick to draw (North Korean leader) Kim Jong-un to the (negotiating) table aimed at freezing North Korea's nuclear weapons program and denuclearization." (Yonhap)