WASHINGTON: US President-elect Joe Biden is to formally introduce his nominees for top national security and foreign policy posts on Tuesday as the White House transition finally gets underway with Donald Trump effectively - if not explicitly - admitting defeat.

The green-lighting of the transition came after Michigan certified a Biden victory on Monday and more members of Trump's Republican Party came out demanding the impasse end.

Trump's long-shot efforts to overturn the November 3 election results received a further blow on Tuesday when Pennsylvania certified that Biden had won the Keystone State. Biden, 78, revealed his picks on Monday for secretary of state, national security advisor, secretary of homeland security, and other top administration posts.

The president-elect and Senator Kamala Harris, the vice president-elect, are to publicly present the nominees to the press at a 1:00 pm (1800 GMT) event in Biden's hometown of Wilmington, Delaware.

Trump, who has made few public appearances since his election defeat, was scheduled to attend a traditional Thanksgiving turkey-pardoning event at the White House.

Biden's public unveiling of his nominees comes after Trump signed off on the start of the White House transition while still refusing to concede the November 3 election.

With Biden having won a comfortable victory, Trump's last card is to try to disrupt the normally routine process of state-by-state certification of results.

Like it has elsewhere, that effort failed in Michigan on Monday and in Pennsylvania on Tuesday, and there appears to be little Trump can do to prevent the Electoral College from meeting on December 14 to certify Biden's victory.