President Donald Trump said Thursday that he would leave the White House if Joe Biden is officially confirmed the winner of the US election, making a further concession of defeat even as he railed against the "rigged" vote.

Trump has made an unprecedented attempt to defy the results of the election by refusing to concede, spreading wild theories about stolen ballots and launching baseless legal challenges that have been thrown out by courts across the country.

Answering his first questions from reporters since the November 3 vote, the president moved closer to accepting that he would serve only one term in office before Biden is inaugurated on January 20.

When asked if he would leave the White House if the Electoral College confirmed Biden's victory, Trump said, "Certainly I will. And you know that."

But "if they do, they made a mistake," he said, adding, "It's going to be a very hard thing to concede."

The Electoral College, which determines the White House winner, will meet on December 14 to certify Biden's victory, with Biden receiving 306 votes to Trump's 232.

"This was a massive fraud," Trump said about the result, again without providing any evidence during his remarks to reporters on Thursday, the Thanksgiving holiday.

He described the US voting infrastructure as "like a third-world country."

Earlier in the day he tweeted that "this was a 100% RIGGED ELECTION," while on Wednesday he called on his Republican supporters "to turn the election over."

Donald Trump
US President Donald Trump has fought against the election results that will end his time in office Photo: AFP / ANDREW CABALLERO-REYNOLDS

President-elect Biden has said that Americans "won't stand" for attempts to derail the outcome, and called for Americans to unite to fight the worsening Covid-19 pandemic.

Trump, who tore up countless norms during his four years in power, has carved out new territory with his refusal to concede to Biden.

Supporters suggest he is already eyeing an announcement of running for president again in 2024.

Trump, 74, is alleging -- among other conspiracy theories -- that voting machines deliberately deleted millions of his votes, though the government election security agency declared it "the most secure" election in US history.

After coming under pressure from a slowly building trickle of senior Republicans, Trump this week ended his blockade of government assistance to ease Biden's preparation for assuming the presidency.

Biden this week also introduced a slate of veteran diplomats and policy-makers who will make up his national security and foreign policy team, saying: "America is back, ready to lead the world."

"These public servants will restore America's global leadership and moral leadership," Biden, 78, said as the six men and women stood behind him wearing face-masks.

Biden said that in his first 100 days in office he will tackle the Covid crisis, scrap Trump policies "damaging" the environment and push legislation offering millions of undocumented residents a route to citizenship.

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