SEOUL (AFP) - US Secretary of State John Kerry slammed North Korea's "unacceptable" rhetoric on Friday, while voicing US support for South Korean moves towards a possible dialogue with Pyongyang.

Speaking to reporters in Seoul, Mr Kerry said North Korean leader Kim Jong Un would be making a "huge mistake" if he pushed ahead with an expected missile launch, and pressed China to exert more leverage over its allies in Pyongyang.

But with military tensions on the Korean peninsula at their highest level for years, Mr Kerry said Washington also chose to "honour" the vision of South Korea's new President Park Geun Hye, who was elected on a pledge of greater engagement with Pyongyang.

"President Park was elected with a different vision for the possibilities of peace and we honour that vision," Mr Kerry said.

"We're prepared to work with conviction that relations between North and South can improve and they can improve very quickly.

"I think we have lowered our rhetoric significantly and we are attempting to find a way for reasonableness to prevail here," he added.

Mr Kerry, making his first trip to Asia as Secretary of State, met earlier with Ms Park, who in recent days has made a series of statements that were seen as offering Pyongyang a dialogue.

On Friday, Yonhap news agency quoted Ms Park as telling ruling party officials that the South should meet with the North and "listen to what North Korea thinks".

There was no indication what form such a meeting might take.

"We want to emphasise that the real goal should not be reinforcing the fact that we will defend our allies - which we will - but it should be emphasising for everybody the possibilities of peace," Mr Kerry said.