In Singapore ahead of a major security forum, Pompeo called for "maintaining diplomatic and economic pressure" against Pyongyang.

His North Korean counterpart, Ri Yong Ho, is also in the city-state for the ASEAN Regional Forum, which takes place later in the day. But Pompeo said he was yet to meet with him on the sidelines of the gathering.

At a landmark summit with President Donald Trump in June, the North's leader Kim Jong Un signed up to a vague commitment to "denuclearisation of the Korean Peninsula" -- a far cry from long-standing US demands for complete, verifiable and irreversible disarmament.

While there have been small signs of progress since the summit, news reports indicate Pyongyang is continuing to build rockets and there have been concerns that some member states are relaxing the enforcement of United Nations sanctions on the North.