US Secretary of State Rex Tillerson said Thursday that two decades of efforts to stop North Korea's nuclear ambitions have not worked and that a new approach is needed.

"I think it's important to recognise that the political and diplomatic efforts of the past 20 years to bring North Korea to the point of denuclearisation have failed," he told a press conference with Japan's foreign minister.

"In the face of the ever escalating threat it is clear that a new approach is required."