US President Barack Obama said on Friday that Donald Trump's recent comments that South Korea and Japan should acquire nuclear weapons show the leading Republican presidential candidate is not well informed on international relations.

Trump told The New York Times in an interview published on Sunday that he would consider letting the countries build their own nuclear weapons, rather than having them count on US protection against North Korea and China.

Obama has made no secret of his disdain for the billionaire businessman, and he continued that theme with his repudiation of Trump's pronouncements.

"What do the statements you mentioned tell us?" Obama told reporters at a news conference at the conclusion of a nuclear security summit. "They tell us that the person who made the statements doesn't know much about foreign policy or nuclear policy or the Korean peninsula, or the world generally."

Obama said Trump's candidacy had come up as a topic with world leaders on the sidelines of the summit, and he lamented the state of the Republican race.

"Even ... those countries that are used to a carnival atmosphere in their own politics want sobriety and clarity when it comes to US elections because they understand the president of the United States needs to know what's going on around the world," he said.

Obama took apart Trump's positions point by point.

The US alliance with Japan and South Korea was one of the cornerstones of America's presence in the Asia Pacific region and had underwritten peace and prosperity there, he said.

The relationship had been a boon to US commerce and influence in addition to preventing nuclear escalation and conflict, he added.

"You don't mess with that. It is an investment that rests on the sacrifices that our men and women made back in World War II when they were fighting throughout the Pacific," Obama said.

"We don't want somebody in the Oval Office who doesn't recognize how important that is." – Reuters