Democratic candidate Joe Biden has consolidated his lead over Donald Trump in Pennsylvania, a key state in the US presidential race, results data show.
Mr Biden is ahead by nearly 29,000 votes, with 99% counted. If he takes the state, he will win the election.
Earlier, Mr Biden edged ahead of his Republican rival in Georgia, another important battleground state, where a recount will now be held.
Meanwhile, the Trump campaign said: "This election is not over."
Mr Biden is expected to address the nation from his campaign headquarters of Wilmington, Delaware, on Friday evening about his growing leads. His team is monitoring news networks to see whether they will declare him the winner.
If the Democrat is declared the victor this weekend, the transition process is expected to begin on Monday.
The Secret Service has sent reinforcements to Delaware to beef up Mr Biden's security detail. The Federal Aviation Administration has restricted flights over Wilmington's airspace.
However, there is no indication Mr Trump will concede to his opponent in the short term.
"Joe Biden should not wrongfully claim the office of the President," he tweeted on Friday afternoon. "I could make that claim also. Legal proceedings are just now beginning!"
Mr Trump has been making unsubstantiated claims of election fraud, spurring some fellow Republicans to speak up that the rhetoric should be toned down.
What's the current state of the race?
Mr Biden is leading Mr Trump by more than 4 million votes out of a record 145 million cast.
But US presidential election results are decided on a state-by-state basis in the Electoral College, and the contest is much closer in the key battlegrounds.
Mr Biden has 253 Electoral College votes, while Mr Trump has 214. To win the White House, a candidate needs 270.
Some news organisations have a higher tally for Mr Biden, having projected a win for the Democrat in Arizona. But the BBC considers the state too early to call.
Pennsylvania, where Mr Biden was born, has 20 Electoral College votes. If the Democrat wins it, he would secure the victory with 273 votes.
The Rust Belt state voted Democratic in six consecutive White House races before it swung to Mr Trump in 2016.
In Georgia, Mr Biden is currently leading with more than 4,000 votes, and 99% of the ballots counted. Georgia's secretary of state said there would be a recount because the margin was so small.
Georgia (16 electoral votes) is a traditionally Republican state and has not been won by a Democrat in a presidential race since 1992.
Mr Biden leads by more than 22,000 votes in Nevada (six electoral votes) and by fewer than 30,000 in Arizona (11 electoral votes).
Mr Trump leads in North Carolina (15 electoral votes) by more than 76,000 ballots.
Mr Trump's team says legal challenges and recounts in some states will favour them.
Bob Bauer, a Biden campaign lawyer, says the lawsuits are legally "meritless".
But on Thursday, a judge in Pennsylvania ruled that Republican election observers should be granted close-up observation of the counting.
A win in just Pennsylvania or two of the other four remaining states would be enough to confirm Mr Biden as president-elect.
Mr Trump would need to win Pennsylvania and three of the remaining four states.
It won't be long now.
Overnight, Joe Biden pulled ahead of Donald Trump in Georgia. And with the latest batch of vote totals released, Biden has taken the lead over Trump in Pennsylvania.
Georgia's 16 electoral votes would be enough to guarantee at least a tie in the Electoral College, pending results from the other uncalled states. Pennsylvania would single-handedly deliver the White House to Biden.
The trendlines in both states are clear, as well. Biden has been netting more votes with each new tally, and he's only going to extend his lead in both states. There are fewer ballots to count in Georgia, so the state is going to stay close - and the recount has already been announced.
By the time all is said and done, however, Biden should lead in Pennsylvania by more than Donald Trump won the state by in 2016.
The drama from these last few days is the result of the process by which these states have counted their votes - tallying mailed-in ballots that have tilted heavily toward Biden after in-person ones, which have been in Trump's favour.
If the process had been reversed, the picture on election night would have been much clearer from the start.
After the dust settles, the results are what they are. And in Delaware, Biden is probably smiling.
What's the mood inside the White House?
President Trump is angry and disappointed that more of his allies are not rallying to his side on television or in the streets, according to White House officials.
He has been watching television, making phone calls to on-the-ground campaign offices, and dividing his time between the Oval Office and the residence.
Several aides did not show up for work on Friday and the White House was described as "very empty" with a sombre mood.
The president has indicated to senior advisers that he will forge ahead with legal challenges to the results, although there is still no strategy for such litigation.
One senior outside adviser to the president described his mood on Friday as "somewhere between sullen and hopeful".
The source added that Mr Trump "was the last one to think he could win in 2016", despite his top aides telling him that he would.