Joe Biden has pulled ahead of Donald Trump in Pennsylvania, a key state in the presidential race, results data show.
The Democratic candidate is leading by more than 13,000 votes, with 98% counted. If Joe Biden takes the state, he will win the election.
Earlier, Joe Biden edged ahead of his Republican rival in Georgia, another key battleground state, where a recount will now be held.
Meanwhile, the Trump campaign said: “This election is not over.”
Trump campaign lawyer Matt Morgan claimed without evidence that ballots in Georgia were “improperly harvested” and that in Pennsylvania election observers were not given “meaningful access” to counting, despite a judge’s order allowing them further access on November 5.
In a statement later, President Trump said: “From the beginning we have said that all legal ballots must be counted and all illegal ballots should not be counted, yet we have met resistance to this basic principle by Democrats at every turn.
“We will pursue this process through every aspect of the law to guarantee that the American people have confidence in our government. I will never give up fighting for you and our nation.”
A senior Trump administration official has told CBS News President Trump does not plan to concede in the event of a victory declaration by Joe Biden.
There are reports from the Biden campaign in Delaware that he will make a primetime address to the nation on Friday evening local time. This is believed to be dependent on the race being called by then.
Joe Biden currently has 253 Electoral College votes, while Donald Trump has 214. To win the White House, a candidate needs 270.
Some news organizations have a higher tally for Mr Biden, having projected a win for the Democrat in Arizona.
Pennsylvania, where Joe Biden was born, has 20 Electoral College votes. If the Democrat wins it, he will secure the victory with 273 votes.
Election officials there said the count could take several days.
Pennsylvania has always been a major political battleground. The state voted Democrat in six consecutive races before it swung to Donald Trump in 2016.
In Georgia, Joe Biden is currently leading with more than 1,500 votes, with 99% of the ballots counted. Georgia’s secretary of state said there would be a recount because the margin was so small.
Georgia is a traditionally Republican state and has not been won by a Democrat in a presidential race since 1992.
President Trump’s team says legal challenges and recounts in some states will favor them.
Bob Bauer, a Biden campaign lawyer, says the lawsuits are legally “meritless” and designed “to message falsely about what’s taking place in the electoral process”.
The vote is also currently too close to call in Nevada and North Carolina.
A win in just Pennsylvania, or two of the other four remaining states would be enough to confirm Joe Biden as president-elect.
Donald Trump, meanwhile, would need to win Pennsylvania and three of the remaining four states.
He has cut the Democratic candidate’s lead in Arizona (11 electoral votes) to less than 44,000 votes, with 93% counted.
He also had a lead of more than 76,000 in North Carolina (15 electoral votes), with 96% of votes tallied.
In Nevada, Joe Biden has an edge of more than 20,000 over Donald Trump. The state has six votes under the electoral college system. An election official there said the results from more than 51,000 postal ballots would be updated on November 6.
President Trump has made unsubstantiated claims of election fraud.
Speaking from the White House on November 5, the president said: “If you count the legal votes, I easily win. If you count the illegal votes they can try to steal the election from us.”
Beyond allegations of irregularities, the Trump campaign has not presented any evidence.
President Trump added: “We were winning in all the key locations, by a lot actually, and then our numbers started getting miraculously whittled away in secret.”
He actively discouraged his supporters from voting by mail, while Joe Biden urged his voters to do so, and it is these postal ballots that are now being tallied in the key states.
Election analysts also say President Trump’s claims of Democratic electoral corruption are undermined by the better-than-expected performance of his fellow Republicans in congressional races across the map.