WASHINGTON (REUTERS) - US President Donald Trump on Sunday (Nov 15) briefly acknowledged losing the United States election in a morning Twitter post but then backtracked, saying he concedes "nothing", and vowing to keep up a court fight that election-law experts say is unlikely to succeed.

President-elect Joe Biden focused on the coronavirus pandemic, a crisis that is intensifying in the weeks before he takes office, setting meetings with vaccine developers.

Mr Ron Klain, Mr Biden's pick for White House chief of staff, urged Trump's administration to allow a seamless transition, calling it vital for national security and combating Covid-19.

The total number of US coronavirus cases passed 11 million on Sunday, according to Reuters data, a million more new cases from just a week ago, the fastest increase since the pandemic began.

The health crisis will be a paramount concern for Mr Biden, who takes office on Jan 20. Mr Klain said Mr Biden's scientific advisers would meet Pfizer Inc and other drugmakers starting this week to prepare for the "giant logistical project" of widespread vaccination against a virus that has killed more than 245,000 Americans and thrown millions more out of work.

Mr Biden defeated Mr Trump in the Nov 3 election by winning a series of battleground states the Republican incumbent won in 2016. The Democratic former vice-president also won the national popular vote by at least 5.5 million votes, or 3.6 percentage points, with some ballots still being counted.

Mr Trump, pursuing long-shot litigation contesting election results in several states, made conflicting statements on Twitter.

He initially appeared to admit for the first time publicly that Mr Biden won, then reversed course. He also repeated unfounded claims of widespread voter fraud.

"He won because the Election was Rigged," he wrote, not referring to Mr Biden by name, adding a list of complaints about vote counting. About 90 minutes later, he wrote: "He only won in the eyes of the FAKE NEWS MEDIA. I concede NOTHING!"

After a golf outing at a club he owns in Virginia, Mr Trump said on Twitter he would soon file "big cases showing the unconstitutionality of the 2020 Election". His campaign has filed lawsuits seeking to overturn the results in multiple states, without success.

Legal experts have said the Trump litigation stands little chance of altering the election's outcome, and election officials of both parties have said there is no evidence of major irregularities.

Speaking on NBC's Meet The Press, Mr Klain said: "Donald Trump's Twitter feed doesn't make Joe Biden president or not president. The American people did that."

The decision by the General Services Administration, headed by a Trump appointee, not to recognise Mr Biden as President-elect has prevented Mr Biden and his team from gaining access to government office space and funding normally afforded to an incoming administration to ensure a smooth transition.

Without explicitly mentioning the transition, Mr Trump praised the agency's administrator, Ms Emily Murphy, writing on Twitter: "Great job Emily!"

Mr Klain said Ms Murphy's agency must formally recognise Mr Biden, saying it is critical to ensure the President-elect receives intelligence briefings before taking office and to facilitate coordination with the White House coronavirus task force.

He urged Congress to pass bipartisan coronavirus relief legislation by year's end. Talks on such legislation have stalled.

'Vaccine challenge'

Mr Klain previously said a smooth transition was necessary to ensure the government was prepared to roll out a Covid-19 vaccine early next year.

Pfizer said last week its vaccine candidate proved more than 90 per cent effective in initial trials, giving hope that widespread vaccination in the coming months could help get the pandemic under control.

Other companies also are in advanced stages of developing promising vaccines.

"It's great to have a vaccine, but vaccines don't save lives. Vaccinations save lives," Mr Klain said. "And that means you've got to get that vaccine into people's arms all over this country."

The mechanics of manufacturing and distribution lie with the US Department of Health and Human Services, he added, making it important for Mr Biden's team to be able to coordinate with current HHS officials.

Speaking on CNN's State Of The Union, Dr Anthony Fauci, the top US infectious diseases expert and a White House task force member, underscored the importance of a smooth transition in pandemic control efforts.

"It would be better if we could start working with them," he said of coordination between the existing task force and Mr Biden's team.

Dr Fauci welcomed Mr Klain's selection, saying he had been "terrific" under former president Barack Obama in managing the US response to a 2014 Ebola outbreak.

Mr Biden's campaign said it had raised US$10.5 million (S$14.1 million) of its target of US$30 million for a "Biden Fight Fund" aimed at election defence efforts, according to a fund-raising plea sent on Sunday.

A laborious hand recount is under way in Georgia, where Mr Biden has been projected the winner and holds a lead of more than 14,000 votes.

Mr Patrick Moore, a Biden campaign legal adviser, said the recount had so far shifted vote totals "almost imperceptibly", and in Mr Biden's favor, and there had been no evidence of widespread irregularities.

Control of the US Senate will be decided by two January run-off elections in Georgia, which will be important for the fate of Mr Biden's ambitious legislative agenda. Mr Klain said Mr Biden may campaign in Georgia ahead of the run-offs.