North Korea's Kim Jong Un understands that denuclearization must happen 'quickly', US Secretary of State Mike Pompeo said on Thursday, warning there will be no sanctions relief for Pyongyang until the process is complete.

Pompeo said Washington remained committed to the 'complete, verifiable and irreversible' denuclearization of North Korea, after the joint statement from the US-North Korea summit in Singapore drew criticism for lack of detail on the key issue.