A key South Korean diplomat says Seoul, Washington and Tokyo will come up with an unprecedented response to a seventh nuclear test by North Korea.

The official on Thursday briefed South Korean reporters in Tokyo on the result of the meeting between the chief nuclear envoys of the three sides held the previous day in Japan.

While noting that the joint response will be unlike any other previously seen, the official added that the international community is certainly expected to sternly deal with North Korea should it push ahead with another underground nuclear detonation.

The South Korean official said that the three sides' chief nuclear negotiators discussed specific response measures to the contingency during the meeting the day before.

The official noted that a nuclear test by the North is a violation of UN Security Council resolutions and a grave challenge to the global nonproliferation regime.

The official also declined to predict a possible date for a nuclear test while confirming that the North’s preparations have been complete since May.