The key battleground state of Wisconsin has been called for Joe Biden in America's knife-edge election, as President Donald Trump's team pushed for a recount.
The president also instigated legal action over ballot counting in neighbouring must-win Michigan.
Media outlet AP declared Wisconsin had narrowly gone for Mr Biden by some 20,000 votes.
President Trump's campaign manager has said he will "immediately" request a recount in Wisconsin.
Bill Stepien said in a statement: "The president is well within the threshold to request a recount and we will immediately do so."
Neither candidate has yet achieved the 270 electoral college votes necessary to win and counting was still under way in the states which will decide the outcome.
Mr Trump's campaign has taken legal action over ballots being counted in Michigan, according to Mr Stepien.
He claimed the Trump campaign "had not been provided with meaningful access to numerous counting locations to observe the opening of ballots and the counting process, as guaranteed by Michigan law".
He added: "We have filed suit today in the Michigan Court of Claims to halt counting until meaningful access has been granted. We also demand to review those ballots which were opened and counted while we did not have meaningful access.
"President Trump is committed to ensuring that all legal votes are counted in Michigan and everywhere else."
The events came as Mr Trump continued to question the legitimacy of the election, while Mr Biden's campaign claimed the Democrat challenger was on course for the White House.
President Trump had earlier falsely claimed victory and threatened to go to the US Supreme Court, as he warned that a "fraud on the American nation" was being carried out over the way votes were being counted.
Mr Biden's campaign said the president's extraordinary comments, made in the White House against a backdrop of US flags, were a "naked attempt to take away the democratic rights of American citizens".
And the Democrat challenger's campaign team said Mr Biden was on course to take key battleground states, providing him with a path to the presidency.
But Mr Trump's team insisted that the president would secure a second term, including through a shock comeback in Arizona, a state which major news organisations have already called for Mr Biden.
In a dramatic statement in the White House, Mr Trump said Tuesday's election, which was characterised by a high number of mail-in and early votes, in part due to the coronavirus crisis, had been "an embarrassment to our country".
"We were getting ready to win this election – frankly we did win this election," Mr Trump said.
The president announced that "we will be going to the US Supreme Court, we want all voting to stop".
Later on Wednesday he claimed that it was "very strange" that "surprise ballot dumps" had eroded his overnight lead in key states.
"How come every time they count mail-in ballot dumps they are so devastating in their percentage and power of destruction?" he said.
Mr Trump has nominated three of the Supreme Court's nine justices – including, controversially, Amy Coney Barrett, whose appointment was confirmed just a week before the election.
It is unclear what, if any, legal basis the president would have, with Mr Biden's camp insisting that the law required every "duly cast vote" to be counted.
Mr Biden said: "We won't rest until everyone's vote is counted."
Last night I was leading, often solidly, in many key States, in almost all instances Democrat run & controlled. Then, one by one, they started to magically disappear as surprise ballot dumps were counted. VERY STRANGE, and the "pollsters" got it completely & historically wrong!
— Donald J. Trump (@realDonaldTrump) November 4, 2020
His campaign had been braced for Mr Trump to seize on record numbers of postal votes to allege he was being cheated.
Mr Biden's campaign chief Jen O'Malley Dillon said the Democrat was on a "clear path to victory" and would "garner more votes than any presidential candidate in history".
Wisconsin, Michigan, Pennsylvania and Nevada would all go to Biden, she claimed.
Among the remaining undeclared states, Georgia is a "toss-up" and North Carolina is "really tight" but "probably leaning towards Trump right now".
Ms O'Malley Dillon said: "Last night the president of the United States falsely claimed that he had won this race and then demanded that votes stop being counted.
"The American people get to pick their president, the president does not get to pick the people whose votes get counted."
Further results in Nevada, where the two candidates are neck and neck, will not be announced until Thursday, leaving six college votes up for grabs.
Mr Trump, however, has seized major victories in Florida and Ohio, which both have long records of choosing the winner of the White House.
Despite the Trump campaign's claims, former vice president Mr Biden is believed to have won Arizona, a state that has only backed a Democrat in the race once in 72 years.
Florida's 29 electoral college votes were a must-win for Mr Trump to reach the 270 required for victory, with no Republican having won the White House without the Sunshine State's support since 1924.
It has backed the winner in every election since 1996 and has only gone with the losing candidate twice since 1928.
Ohio is also a significant victory for Mr Trump, having been key to his chances of remaining in the White House, and whoever has won the state has gone on to take the presidency since 1964.
Mr Trump also held on to Texas and its 38 college votes in a fiercely-contested battle.
Mr Biden has painted the election as the "battle for the soul" of the nation, saying democracy itself is at stake. Mr Trump reprised his "Make America Great Again" mantra during the bitter campaign.
Economic fairness and racial justice have been prominent issues in the election race.
Both men have also clashed over the Covid-19 response, as the nation reels from more than 230,000 coronavirus deaths and millions of job losses.
Steady queues of voters flocked to the polls on Tuesday after about 100 million Americans voted early, setting the nation on course for a record turnout figure.
Each state gets a number of electoral college votes roughly in line with its population and they largely hand them all to the winner in that state.
With 538 up for grabs across the country, 270 is the winning number – a target which remains within reach for either candidate, depending on how the results unfold over the coming hours and days.
In the UK, Prime Minister Boris Johnson refused to criticise Mr Trump's actions in threatening court action, telling MPs: "We don't comment as a UK Government on the democratic processes of our friends and allies."