WASHINGTON (AFP) – President Barack Obama scathingly dismissed White House hopeful Donald Trump's foreign policy proposals on Friday and warned that the world was watching the upcoming US election.

At the end of a summit on nuclear security that Obama hosted in Washington, he was asked about Trump's suggestion that Japan and South Korea develop their own nuclear weapons.

"The statements you mentioned, what do they tell us?" Obama demanded, rhetorically.

"They tell us the person who made the statements doesn't know much about foreign policy or nuclear policy or the Korean peninsula or the world generally."

Trump, the Republican frontrunner, says he wants US allies to pay for more of their own defense and allow costly American forces to disengage from their regions.

Obama damned this as naive and an abdication of American leadership that would upset close allies and make the world a more dangerous place.

"I've said before that, you know, people pay attention to American elections. What we do is really important to the rest of the world," he said.

"Our alliance with Japan and the Republic of Korea is one of the foundations, one of the cornerstones of our presence in the Asia-Pacific region," he said.

"It has underwritten the peace and prosperity of that region. It has been an enormous boon to American commerce and American influence," he said.

The US alliance has brought peace to countries that had fought fierce wars in the past.

"So you don't mess with that. It's 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.

"And we don't want somebody in the Oval Office who doesn't recognize how important that is."

Obama added that concern about Trump's comments had come up on the sidelines of his summit meetings with world leaders.