Donald Trump, the GOP's presumptive presidential candidate, will announce his pick for vice president in New York on Friday, he said on Twitter, as speculation runs rampant over a handful of potential running mates.

Among those believed to be at the top of the Republican's list are Indiana Governor Mike Pence, New Jersey Governor Chris Christie and former speaker of the House Newt Gingrich.

"I will be making the announcement of my Vice Presidential pick on Friday at 11am in Manhattan. Details to follow," the 70-year-old real estate mogul posted on Twitter late on Wednesday.

In an interview earlier in the day with Fox News, Mr Trump said that he had narrowed the field down to "three, potentially four, but in my own mind, I probably am thinking about two".