Donald Trump chose a Republican critic to join his cabinet Wednesday, the first woman on his top team after his early picks rewarded campaign loyalists.

Trump's nomination of South Carolina's 44-year-old governor, Nikki Haley, as US ambassador to the United Nations will be seen as a sign he is ready to broaden his political base.

It came amid reports that he is also considering former Massachusetts governor Mitt Romney, another harsh campaign critic, this time for the key post of secretary of state.

And it followed a cordial chat between Trump and the New York Times, a newspaper he considers hostile, in which he softened his stance on climate change, torture and prosecuting his defeated rival Hillary Clinton.

The 70-year-old property tycoon told the Times that he is "seriously, seriously considering" appointing widely-respected retired Marine general James Mattis as his defense secretary.