A top U.S. government adviser who has in the past been granted unprecedented access to North Korea's nuclear weapons program is warning that the "quick" denuclearization process sought by the Trump administration could, in fact, take far longer, possibly up to 10 years, according to an analysis by experts.

Siegfried Hecker, a former director of the Los Alamos weapons laboratory in New Mexico who is currently a professor at Stanford University in California, argues in a report released late Monday that the best the U.S. can hope for is a phased approach to dismantling the North's nuclear weapons program that targets the most dangerous parts first.

But such an approach would be at odds with U.S. President Donald Trump's demand that the North immediately and completely relinquish his arsenal without American concessions in return.