US President-elect Joe Biden presents his security and foreign policy team

US President-elect Joe Biden introduced his nominees for top national security and foreign policy posts on Tuesday, saying: "America is back, ready to lead the world."

Biden, 78, at an event in his hometown of Wilmington, Delaware, presented his picks for secretary of state, national security advisor, homeland security secretary, head of national intelligence, UN ambassador and climate change envoy.

The names of his selections were revealed by the Biden team on Monday and the former vice-president appeared on stage with them at a socially distanced event at the Queen Theater on Tuesday.

"It is a team that will keep our country and our people safe and secure," Biden said. "It is a team that reflects the fact that America is back, ready to lead the world, not retreat from it." 

Biden said that after he is inaugurated on January 20, 2021, the United States would "once again sit at the head of the table, ready to confront our adversaries and not reject our allies."

Biden's remarks came shortly after US President Donald Trump suffered another setback in his unprecedented efforts to overturn the results of a US election with unsubstantiated claims of fraud.

Pennsylvania certified the November 3 election results on Tuesday, a day after the state of Michigan did so, a move which triggered the General Services Administration (GSA) to launch the transition process.

As more members of his Republican Party and business leaders came out demanding an end to the impasse, Trump signed off on the GSA move, effectively admitting defeat while still refusing to concede.

Shortly before the Biden event was to begin, Trump appeared in the White House briefing room to make brief unscheduled remarks.

Trump, who has made few public appearances since his defeat, did not address the vote but touted the Dow Jones Industrial Average surging past the 30,000-point level for the first time ever.

"Despite everything that's taken place with the pandemic, I'm very thrilled with what's happened on the vaccine front, that's been absolutely incredible," Trump said.

"But the stock market has just broken 30,000, never been broken that number, that's a sacred number, 30,000, nobody thought they'd ever see it," he said.

Trump, who was scheduled to attend a traditional Thanksgiving turkey-pardoning event at the White House later in the day, left without responding to shouted questions from reporters.

'I concede NOTHING!!!!'

Trump acknowledged on Monday – 16 days after US television networks declared Biden the president-elect – that it was time for the GSA to unblock funding for the Biden transition team.

Trump 74, continued to insist, however, that he would ultimately be declared the winner of the election despite having failed to present any credible evidence of fraud and suffering a string of humiliating defeats in court.

He continued in the same defiant vein on Tuesday.

"The GSA does not determine who the next President of the United States will be," he tweeted.

He also retweeted a picture of himself in the Oval Office with the caption: "I concede NOTHING!!!!!"

The GSA determination that Biden is the apparent winner gives the president-elect access to classified briefings on national security threats and will allow his top aides to coordinate with federal health officials on addressing the worsening Covid-19 pandemic.

Unveiling his cabinet selections, Biden said he had received calls from up to 20 world leaders saying they were "looking forward to the United States reasserting its historic role as a global leader."

"This team meets this moment," he said, as his masked nominees stood on stage behind him. "They embody my core belief that America is strongest when it works with its allies."

The slate unveiled by Biden includes veterans of the Barack Obama administration and signals a return to traditional US diplomacy.

Former State Department number two Antony Blinken was tapped to be secretary of state while Jake Sullivan, who advised Biden when he was vice president, was nominated to be national security advisor.

Biden named the first woman, Avril Haines, as director of national intelligence, and another woman, career diplomat Linda Thomas-Greenfield, to be UN ambassador.

Cuban-born Alejandro Mayorkas was named to head the Department of Homeland Security, whose policing of tough immigration restrictions under Trump was a frequent source of controversy.

Following through on his campaign promise to raise the profile of global warming threats, Biden named former secretary of state John Kerry as a new special envoy on climate issues.

Yellen to Treasury

Former Federal Reserve chairwoman Janet Yellen is expected to be named Treasury Secretary, the first woman to hold the job.

Blinken is expected to spearhead a dismantling of Trump's "America First" policies, including rejoining the Paris climate agreement and the World Health Organisation and resurrecting the Obama-crafted Iran nuclear deal.

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 and Pennsylvania, and there appears to be little Trump can do to prevent the Electoral College from meeting on December 14 to certify Biden's victory.

Biden received 306 votes to Trump's 232 in the Electoral College that determines the White House winner.