Joe Biden and his advisers on Sunday were working on plans to tackle the crises facing a divided America, first and foremost the raging coronavirus pandemic, a day after the Democrat won enough states to clinch the US presidency.

Republican Donald Trump, the first US president to lose a re-election bid in 28 years, gave no sign of conceding, instead pressing ahead with legal fights challenging the outcome.

Top Republicans in Congress likewise had not acknowledged Biden's victory, in a sign of the charged partisan atmosphere he will face when he takes office on Jan. 20, although some members of Trump's party and a bipartisan group focused on the transition urged the president to cooperate.

Biden delivered a message of unity and conciliation in a speech in his home state of Delaware on Saturday, saying it was "time to heal" the nation.

"The work starts right away," Biden Deputy Campaign Manager Kate Bedingfield said on Sunday on NBC's "Meet the Press."

Bedingfield said Biden planned to launch a coronavirus task force on Monday, led by former Surgeon General Vivek Murthy and former Food and Drug Administration Commissioner David Kessler.

More than 237,000 Americans have died of COVID-19 and coronavirus cases have spiked to record numbers in recent days. Some 10 million Americans thrown out of work during coronavirus lockdowns remain idled and federal relief programs have expired.

Biden and his advisers will also move forward with the work of choosing officials to serve in his administration.

Bedingfield added that Biden would "address a mandate to bring the country together - to unify, to lower the temperature, to set aside the harsh rhetoric of the campaign and get to the hard work of governing."

The bipartisan Partnership for Public Service's Center for Presidential Transition said Biden had clearly won and called for the Trump administration to work cooperatively with him.

"History is replete with examples of presidents who emerged from such campaigns to graciously assist their successors," it said in a statement.

Two former senior US intelligence officials - Michael Morell and Avril Haines - have emerged as leading contenders to serve as director of national intelligence or run the CIA under Biden, several current and former intelligence officials said.

Congratulations poured in for Biden from abroad, including from conservative British Prime Minister Boris Johnson, Canadian Prime Minister Justin Trudeau, German Chancellor Angela Merkel and Israeli Prime Minister Benjamin Netanyahu, making it hard for Trump to push his unfounded claims of election fraud.

Saudi Arabia also offered congratulations, more than 24 hours after Biden's victory. The country's de facto leader, Crown Prince Mohammed bin Salman, has enjoyed close relations with Trump, and Biden has said he will reassess ties with the kingdom.

Wearing his trademark red "Make America Great Again" baseball cap, Trump golfed at his course in Sterling, Virginia, for the second day in a row. His motorcade was met by a smattering of admirers and detractors holding signs, including one that read: "Trumpty Dumpty Had A Great Fall."

Unlike other past defeated US presidential candidates, Trump has not made a concession statement or reached out to Biden.

"Since when does the Lamestream Media call who our next president will be?" Trump wrote on Twitter after golfing.

Republican former President George W. Bush said in a statement that he spoke with Biden and congratulated him on his victory.

"Though we have political differences, I know Joe Biden to be a good man, who has won his opportunity to lead and unify our country," Bush said. "The American people can have confidence that this election was fundamentally fair, its integrity will be upheld, and its outcome is clear."

After attending church in Wilmington, Delaware, Biden and his family visited the church's cemetery, where his son Beau and other relatives are buried - as he did on the morning of Election Day on Tuesday.

According to an adviser, Biden plans to repeal a ban on travelers from several Muslim-majority nations, rejoin an international climate accord, reverse Trump's withdrawal from the World Health Organization and buttress a program protecting from deportation "Dreamers" brought to the United States illegally as children.