Joe Biden's campaign warned Friday that President Donald Trump could be escorted from the White House if he refuses to admit defeat in America's knife-edge election.

Democratic challenger Biden is edging towards the presidency after pulling ahead in the key states of Pennsylvania and Georgia.

But Trump has made it clear that he is not ready to concede, launching unsubstantiated claims of voter fraud and claiming falsely that he had been cheated out of re-election.

“As we said on July 19th, the American people will decide this election. And the United States government is perfectly capable of escorting trespassers out of the White House,” said Biden campaign spokesman Andrew Bates.

In an interview with Fox News in July, Trump refused to commit to accepting the results of the election and a peaceful transfer of power if he lost.

With tens of thousands of votes remaining to be counted, many of them from heavily Democratic areas, Biden opened up a 9,000-vote lead over the Republican incumbent in Pennsylvania, real-time state election results showed.

Pennsylvania and its 20 electoral votes would be enough to put the 77-year-old Biden past the magic number of 270 votes in the Electoral College, which determines the presidency.

Biden has also taken a razor-thin lead in Georgia, a state once seen as reliably Republican, which announced Friday that it will recount the votes.

As of late morning there was no word on Biden's movements on Friday.

He last spoke on Thursday afternoon when he told reporters in his home town of Wilmington, Delaware, that he had “no doubt” he would be declared the winner of the election.

With his victory looking increasingly likely, the US Secret Service increased its protective bubble around the former vice president, The Washington Post reported Friday.