Updated 5am Saturday

As he closed in on winning the US presidency on Friday, Democrat Joe Biden said he would waste no time in addressing the COVID-19 pandemic upon taking office.

"I want everyone, everyone to know on day one we're going to put our plan to control this virus into action," Biden said in a late-night address from his hometown Wilmington, in Delaware.

Biden expressed confidence that he would defeat President Donald Trump as vote counting dragged on from Tuesday's election but stopped short of declaring himself the winner.

He pledged to unite a bitterly divided nation.

"It's time for us to come together as a nation to heal," Biden said.

Biden takes the lead in Pennsylvania

Earlier Biden took the lead in key battleground state Pennsylvania, where a victory for the former vice president would push him past the threshold of electoral votes needed to win the White House.

Pennsylvania, and its 20 electoral votes, would be enough to vault the 77-year-old past the magic number of 270 votes in the Electoral College, which determines the presidency.

With some 40,000 votes remaining to be counted in Pennsylvania, many from heavily Democratic areas, Biden opened up a 12,400-vote lead over the Republican incumbent, according to real-time state election results.

Biden currently has at least 253 electoral votes. He is also currently in the lead in key states Georgia and Nevada. 

Romney blasts Trump for claiming election 'rigged'

Republican US Senator Mitt Romney said on Friday that it was wrong for Trump to claim that the election was "rigged, corrupt and stolen" by Democratic rivals.

While the president is "within his rights" to request recounts and seek investigations into alleged voting irregularities, as his supporters claim occurred, "he is wrong to say that the election was rigged, corrupt and stolen," said Romney, an occasional Trump critic.

"Doing so damages the cause of freedom here and around the world, weakens the institutions that lie at the foundation of the Republic, and recklessly inflames destructive and dangerous passions.

Georgia says will recount razor-thin vote

The US state of Georgia said it will recount votes from the election in which Biden has eked out a razor-thin lead over President Donald Trump.

"With a margin that small, there will be a recount in Georgia," Georgia Secretary of State Brad Raffensperger told reporters in Atlanta.

Top Democrat Pelosi calls Biden 'president-elect' 

House Speaker Nancy Pelosi, the top Democrat in Congress, on Friday called Biden the "president-elect" of the US.

"This morning it is clear that the Biden-Harris ticket will win the White House," Pelosi told reporters after Biden overtook President Trump in Pennsylvania.

"President-Elect Biden has a strong mandate to lead," she said.

It is "a happy day for our country. Joe Biden is a unifier because he is determined to bring people together."

Pelosi played down the losses but said that the next House election in 2022 "will be a steeper climb" without Trump on the ballot.

The party of the president nearly always loses seats in Congress in the first midterm election.

Trump erupts as Biden closes in on US presidency

Earlier, Trump erupted in a tirade of unsubstantiated claims that he has been cheated out of winning the US election as vote counting across battleground states showed Democrat Joe Biden steadily closing in on victory.

Biden's momentum towards the White House built further with major media outlets reporting he had overtaken Trump by a razor-thin margin in the crucial battleground state of Georgia.

"They are trying to steal the election," an increasingly isolated Trump said in an extraordinary appearance at the White House on Thursday, two days after polls closed.

Providing no evidence and taking no questions afterwards from reporters, Trump spent nearly 17 minutes making the kind of incendiary statements about the country's democratic process that have never been heard before from a US president.

According to Trump, Democrats were using "illegal votes" to "steal the election from us."

"If you count the legal votes, I easily win," he claimed. "They're trying to rig an election. And we can't let that happen."

Trump repeated those claims in a tweet early on Friday.

His rhetoric came as his campaign aggressively challenged the integrity of the huge number of ballots mailed in rather than cast in person on Election Day.

The big shift to postal ballots this year reflected the desire of voters to avoid risking exposure to COVID-19 in crowded polling stations during a pandemic that has already killed 235,000 Americans.

Mail-in ballots have tilted heavily to Democrats. In the crucial state of Pennsylvania, the Trump campaign moved to stop the counting of votes, which authorities were forbidden from processing before Election Day.

Mixed support for Trump

Several major US television networks cut away from live coverage of Trump's event over concerns of disinformation and there were signs of cracks in support within his Republican Party.

Representative Will Hurd called Trump's call to stop vote-counting "dangerous and wrong," while Rupert Murdoch's long supportive New York Post called Trump's allegations "baseless."

But prominent Republicans rallied behind Trump and signalled that they could challenge the legitimacy of results if the president loses.

"I think everything should be on the table," Senator Lindsey Graham said when asked by Fox News host and Trump loyalist Sean Hannity if Pennsylvania's Republican-led legislature should refuse to certify results.

Biden was just one or at most two battleground states away from securing the majority to take the White House. Trump needed an increasingly unlikely combination of wins in multiple states to stay in power.

Biden, who has promised to heal a country bruised by Trump's extraordinarily polarising four years in power, appealed for "people to stay calm."

"We have no doubt that when the count is finished, Senator (Kamala) Harris and I will be declared the winners," he said in comments to reporters in his hometown of Wilmington, Delaware.

"The process is working," he said. "The count is being completed. And we will know soon."

Protests across country

Trump's campaign insisted that the president has a way to win, citing pockets of Republican support yet to be counted and also alleging mass fraud without providing evidence.

Trump's team fanned out across the battleground states challenging the results in court and his supporters converged outside election offices in several cities.

Outside an election office in Arizona's capital Phoenix, far-right conspiracy theorist Alex Jones roused a heavily armed crowd, shouting on a megaphone about Trump's supposed enemies: "They will be destroyed because America is rising." 

In Las Vegas, Trump backers wearing red "Make America Great Again" hats demanded to see ballots being processed.

Brando Madrigal said he wanted to verify that the votes are "not coming from the people who died with COVID-19, people who are out of state, people who don't have the ability to vote because they don't have the papers."

But while Trump was demanding that counting be halted in Georgia and Pennsylvania - where his lead is narrowing - his supporters and campaign insisted that it continue in Arizona and Nevada, where he is trailing.

Bob Bauer, a lawyer for the Biden campaign, dismissed the slew of lawsuits as "meritless."

"All of this is intended to create a large cloud," Bauer said. "But it's not a very thick cloud. We see through it. So do the courts and so do election officials."