Donald Trump announced Friday that outspoken former New York mayor Rudy Giuliani, a key surrogate on the campaign trail, would not join his incoming administration after he takes office next month.

Giuliani had been a candidate for the most prestigious cabinet job of secretary of state but subsequently came under scrutiny in the US media over business dealings that could pose conflicts of interest.

"Rudy would have been an outstanding member of the cabinet in several roles, but I fully respect and understand his reasons for remaining in the private sector," the president-elect said in a statement.