MIAMI (AFP) - Retired neurosurgeon and one-time Republican presidential candidate Ben Carson is set to endorse Mr Donald Trump, a rival who once mocked him, the billionaire front runner said.

Dr Carson himself told Fox News radio on Thursday that "I'm certainly leaning" towards a Trump endorsement, and an aide confirmed that the two men met recently.

Hours later, Mr Trump said at the Republican presidential debate in Miami that the former candidate, who dropped his bid last week, will endorse him at a press conference on Friday (March 11) in Florida.

Mr Trump said he met on Thursday with Dr Carson, who "is endorsing me, by the way, tomorrow morning."

Florida, a main prize in the 2016 nominations race, votes on Tuesday (March 15), and Mr Trump is ahead there in polling. He is seeking to best Senator Marco Rubio, who sees his home state as a do-or-die contest.

The religiously conservative Carson's outsider campaign soared last year but ultimately lost steam as he stumbled over questions about the veracity of aspects of his compelling life story.

Mr Trump also mercilessly taunted and mocked Dr Carson on the campaign trail, particularly over his claim that he once tried to stab a fellow student but that the boy's belt buckle prevented the knife from breaking the skin.

But Mr Trump later sided with Dr Carson over a dispute in Iowa, when Senator Ted Cruz's campaign wrongly told voters that Dr Carson had dropped out of the race before the state's Feb 1 caucuses.

Dr Carson said he believed the billionaire real estate mogul will be able to put aside his caustic, controversial rhetoric that has become a hallmark of his public appearances.

"There are two Donald Trumps. There's the Donald Trump that you see on television and who gets out in front of big audiences, and there's the Donald Trump behind the scenes," Dr Carson told Fox.

"They're not the same person. One's very much an entertainer, and one is actually a thinking individual."