South Korea's leading presidential contenders all cited the United States and China as the key players to stop North Korea from conducting its sixth nuclear test in their second TV debate Wednesday.

The five candidates for the May 9 election were asked during the debate hosted by public broadcaster KBS what diplomatic leverage the South Korean government has to block another nuclear experiment by the North. The question addressed growing regional tensions caused by Pyongyang's saber-rattling and Washington's show of force.

Front-runner Moon Jae-in of the liberal Democratic Party 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.

Moon indicated that China would be the next interlocutor.

"There's also a need to make it clear to China that in the event the north carries out a nuclear test, the deployment of THAAD is inevitable," he said.

China has strongly protested the deployment of the US Terminal High Altitude Area Defense anti-missile system in South Korea and took retaliatory measures on Korean businesses.

Hong Joon-pyo of the conservative Liberty Korea Party 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 Cheol-soo of the center-left People's Party 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 Seong-min of the splinter conservative Bareun Party 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 Sang-jeung of the progressive Justice Party 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)