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.