President Obama has called Donald Trump to offer his congratulations on the Republican's victory and set the ball rolling on a transition process that will hand power to the billionaire businessman in January.

Mr Obama also called Hillary Clinton to express his "admiration for the strong campaign she waged throughout the country", the White House said.

The current President invited the new President-elect to meet him at the White House on Thursday to discuss the transition of power. Mr Trump will be formally sworn in as the 45th president of the United States on 20 January.

Mr Obama had earlier called Mr Trump while he was speaking to his supporters in New York, and so the President-elect called him back after he left the stage.

Trump campaign manager Kellyanne Conway told NBC News the pair had a "great, thorough conversation" and "resolved to work together".

President Obama will make a public statement outside the White House later on Wednesday in which he is expected to reflect on the election results and urge the US to unite after a bitter and divisive election campaign that saw Mr Trump pull off a shock upset against Hillary Clinton.

The President will then continue working on arrangements for Mr Trump taking office. The Obama Administration has been planning the presidential transition since the beginning of the year and will meet with the newly-elected President-elect and his team on Thursday to brief them on the plans.

The White House said: "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".

Mr Obama campaigned heavily for Mrs Clinton during the latter weeks of the campaign and repeatedly attacked her rival. He told a Cleveland rally: "Donald Trump's closing argument is 'what do you have to lose?' The answer is everything."

"“This is a guy who spent all his time trying to convince everyone he was a global elite. … All he had time for is celebrities. And now suddenly he’s acting like a populist. Come on, man.”