WASHINGTON (AFP) -- As North Korea reportedly continues to build missiles and produce plutonium, a top aide to US President Donald Trump said Sunday that "there's nobody in his administration starry-eyed about the prospects of North Korea actually denuclearising."

National Security Advisor John Bolton said that the "point may well come" when the US concludes that North Korean leader Kim Jong Un is not serious about his promises to denuclearise.

But Bolton, speaking on "Fox News Sunday," said Trump had gone out of his way to make it easy for Kim to comply with promises made when the two men met on June 12 in Singapore.

"The president is giving Kim Jong Un a master class on how to hold a door open for somebody," he said, "and if the North Koreans can't figure out how to walk through it, even the president's fiercest critics will not be able to say it's because he didn't open it wide enough."

Pressed on how long the US was prepared to wait - amid reports about North Korean plutonium production, missile work and sanctions violations - Bolton said that depended on a clear showing of North Korean will.

"If they make a strategic decision to give up nuclear weapons, they can do it within a year," he said. "We are waiting to see evidence that in fact that strategic decision has been made."

Trump has touted his Singapore summit with Kim as a historic breakthrough toward the North's denuclearisation.

But since then, both sides have grumbled about the pace of progress - or lack thereof.

North Korean Foreign Minister Ri Yong Ho said Saturday in Singapore that the US was acting with "alarming" impatience.

And while US Secretary of State Mike Pompeo said Friday that he remained "optimistic," he told senators last week that the North was continuing to produce fissile material, and he has urged other countries to maintain pressure on Pyongyang.