A group of South Korean opposition lawmakers defied President Park Geun-Hye and left for China on Monday to discuss the deployment THAAD.

A group of South Korean opposition lawmakers defied President Park Geun-Hye and left for China on Monday to discuss the deployment of a US anti-missile system that has opened a damaging rift between Seoul and Beijing.

Park had urged the MPs to scrap their trip, arguing that it would boost China's opposition to the Terminal High Altitude Area Defense (THAAD) system and deepen divisions in South Korea over the issue.

Seoul's decision to host a THAAD battery, to counter a growing threat from North Korea's nuclear and missile programs, has been condemned by China as a threat to its own security interests and to regional stability.

The ongoing row is threatening to undo the substantial effort President Park has put into strengthening ties with China, which is not only South Korea's largest trade partner but also the key player in curbing North Korea's nuclear ambitions.

Kim Young-Ho, one of the six lawmakers from the main opposition Minjoo Party travelling to Beijing, said their sole motivation was to calm the situation.

"We are visiting with the hope of offering at least a little warmth to the icy Seoul-Beijing ties," Kim was quoted as saying by the Yonhap news agency before leaving Seoul.

But Park insisted the visit would be counter-productive and suggested the MPs were being irresponsibly disloyal.

"On a matter of national security, there should be no division between the ruling and the opposition parties," she told a meeting of top aides Monday morning.

"Some politicians are making ridiculous claims that the deployment of THAAD will only offer good excuses for North Korea to stage more provocations -- an argument that is very similar to the North's own views," she said in comments published on her official website.

THAAD has been the subject of domestic protests in South Korea, particularly by those living in the rural county of Seongju where the first battery will be installed.

Residents say the system's powerful radar poses health and environmental hazards and argue that its presence will make them a key military target.

Opposition parties have been less than supportive of the deployment, although outright criticism has largely been limited to left-leaning MPs and activists.

Responding to Park's criticisms, Kim said calling off the visit at this stage would make things worse.

"It would look like the president had blocked it," he told reporters.

On Sunday, Park's office had issued a statement that chided China for over-reacting to the THAAD deployment.

"We believe that China, before taking issue with our purely defensive move, should raise the issue more strongly with the North," the statement said, referring to a recent series of provocative North Korean missile tests.