US President Barack Obama hammered home his belief that Republican White House hopeful Donald Trump would not be elected, knocking his reality show past and penchant for drawing media attention.

Obama did not limit his criticism to the billionaire real estate tycoon, hitting out at "troubling" statements from the entire GOP field of candidates seeking to replace him.

But he reserved his toughest remarks for Trump, offering a scathing assessment of why he thinks the American people will not elect him.

"I continue to believe that Mr Trump will not be president. And the reason is because I have a lot of faith in the American people. And I think they recognize that being president is a serious job," he told reporters in California yesterday.

"It's not hosting a talk show or a reality show. It's not promotion. It's not marketing. It's hard," he said on the sidelines of a summit with leaders and representatives of 10 Southeast Asian nations.

"It's not a matter of pandering and doing whatever will get you in the news on a given day. And sometimes it requires you making hard decisions, even when people don't like it," Obama continued.

He also noted the need to be "able to work with leaders around the world in a way that reflects the importance of the office."

"During primaries, people vent and they express themselves," Obama said, alluding to Trump's brash, take-no-prisoners style. "Oftentimes it's reported just like entertainment, but as you get closer, the reality has a way of intruding."

"The American people are pretty sensible. And I think they'll make a sensible choice in the end," he concluded.

The 69-year-old Trump has long been ahead in the Republican race for the White House nomination, according to opinion polls.

He lost the Iowa caucuses to Senator Ted Cruz early this month, but roared to victory in the New Hampshire primary last week. He holds a commanding 16-point lead over Cruz in South Carolina, according to a CNN poll. Obama, whose successor will be chosen on November 8, insisted that Trump was not alone in expressing unsettling proposals on the Republican campaign trail.

"He may up the ante in anti-Muslim sentiment, but if you look at what the other Republican candidates have said, that's pretty troubling, too," he said.

"They're all denying climate change. I think that's troubling to the international community," he added.