Democrat Hillary Clinton conceded the 2016 US White House race to Republican Donald Trump and offered to work with the president-elect, who she hoped would be a successful leader for all Americans.

Clinton, appearing at midday in New York after a bruising election loss to the real estate magnate, urged supporters to keep an open mind on Trump and give him a chance to lead.

"Last night I congratulated Donald Trump and offered to work with him on behalf of our country. I hope that he will be a successful president for all Americans," Clinton told hundreds of supporters and staff at a Manhattan hotel.

"This is not the outcome we wanted or we worked so hard for, and I'm sorry that we did not win this election for the values we shared and the vision we hold for our country," she said to cheers.

Possibly facing her last opportunity for a presidential run, Clinton, 69, acknowledged the results were painful and that she was disappointed.

She urged her assembled staff and supporters, deflated after recent national opinion polls indicated a good chance at victory, to continue to work for a better nation.

"This is painful and it will be for a long time," Clinton said. "But I want you to remember this: Our campaign was never about one person or even one election. It was about the country we love and about building an America that's hopeful, inclusive and big-hearted."

The former first lady, US senator and secretary of state said the election results showed the nation was deeply divided, but the voters had spoken.

"Donald Trump is going to be our president. We owe him an open mind and the chance to lead."

President Barack Obama, who campaigned hard against Trump, invited Trump to the White House for a meeting on Thursday (Friday AEST). Obama is due to speak later on Wednesday (Thursday AEST) about the election.

"Ensuring a smooth transition of power is one of the top priorities the President identified at the beginning of the year and a meeting with the President-elect is the next step," the White House said.