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

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

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

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“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 recognise that being president is a serious job,” he told reporters in California.

“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 the Association of South East Asian Nations (ASEAN).

“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.”

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,” Mr Obama said, alluding to Mr 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.”

Mr Trump, 69, 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 Mr Cruz in South Carolina, according to a CNN poll.

Mr Obama, whose successor will be chosen on November 8, insisted that Mr 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.”

No backing down on Supreme Court replacement

In another challenge to his political rivals, Mr Obama again insisted he would nominate a successor to late Supreme Court justice Antonin Scalia, and said Republicans were duty-bound not to hold up the process.

The sudden death of Mr Scalia has become a hot-button election issue.

Almost as soon as the justice’s death was announced, Republican Senate majority leader Mitch McConnell warned that the next president, not Mr Obama, should name the ultra-conservative justice’s replacement.

The choice of a successor to Mr Scalia could tip the balance on the high court — which effectively is left with four liberal-leaning justices and four conservative justices — and affect several major cases on its docket.

“The Constitution is pretty clear about what is supposed to happen now. When there is a vacancy on the Supreme Court, the president of the United States is to nominate someone,” Mr Obama said.

He lamented the “venom and rancor in Washington,” which he said had “prevented us from getting basic work done,” adding: “This would be a good moment for us to rise above that.”