President-elect Donald Trump said Thursday that he will nominate US Marine retired general James Mattis, who oversaw US war efforts in the Middle East, to be his defense secretary.

"We are going to appoint 'Mad Dog' Mattis as our secretary of defense," Trump told a large crowd in Cincinnati, Ohio, referring by nickname to the tough-talking general who headed the US Central Command, giving him authority over troops in Iraq and Afghanistan.

"He's our best. They say he's the closest thing to (World War II-era) general George Patton that we have," Trump said, apparently divulging his pick days ahead of schedule as his transition team had already said there would be no more cabinet announcements this week.

"Okay. So I gave up a little secret. My people over there are probably saying, 'You weren't supposed to do that, Mr Trump'," the president-elect quipped.