US President Donald Trump said Thursday for the first time that he would leave the White House if Joe Biden is officially confirmed the winner of the US election, 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 theories about stolen ballots and launching legal challenges, some of which have been thrown out by courts.

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."

"I think that there will be a lot of things happening between now and (January) 20th," he said.

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 election was a fraud," Trump said, again without providing any evidence during his remarks to reporters at the White House after he spoke to military personnel via video-link on 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."

Trump’s legal team is challenging the election results. This week they held a press conference in Pennsylvania in which they provided a number of examples of alleged electoral fraud and malpractice in the state.

Trump, 74, is alleging - among other 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, a claim Trump takes issue with.

Under pressure from some senior Republicans, Trump this week ended his blockade of government assistance to ease Biden's preparation for assuming the presidency.

Trump said Thursday he would soon travel to Georgia to campaign ahead of two key runoff elections that will decide which party controls the Senate.

The US media has declared Biden president-elect but his position has yet to be confirmed.

AFP/Mizzima