South Korea's foreign minister is warning North Korea that the cost of keeping and testing nuclear weapons will be so high that it could threaten the survival of Kim Jong Un's regime.

Yun Byung-se told the International Peace Institute on Tuesday that the North will pay "the heaviest price" in new sanctions if it defies the international community and goes ahead with a new nuclear test.

If it does, he said the United Nations Security Council must fill all the loopholes in the four rounds of sanctions it has imposed on the North over its nuclear and ballistic missile programs — and individual countries must take similar tough measures.

At the same time, Yun reiterated President Park Geun-hye's proposals last month to reunify Korea, saying "we hope North Korea will respond positively."