US President Barack Obama hammered home his belief on Tuesday 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 people in the US 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 said.

"It's not hosting a talk show or a reality show. It's not promotion. It's not marketing. It's hard," he said.

"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.

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

'A great compliment'

He said Obama's prediction "actually is a great compliment".

Trump outlined his complaints about Obama's presidency, saying: "You look at our budgets, you look at our spending, we can't beat Islamic State. Obamacare is terrible. ... Our borders are like Swiss cheese."

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 Nov 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.

AFP - AP