Joe Biden is now projected to have 284 Electoral College votes, the Associated Press (AP) news agency said on Saturday, above the threshold needed to clinch the hard-fought United States presidential election.

The president-elect was projected to have surpassed the 270 electoral vote-threshold after the AP called Pennsylvania in his favour.

US President Donald Trump did not immediately acknowledge the AP's projection and his campaign has vowed to challenge the result, particularly in the states of Georgia, Pennsylvania, Arizona and Michigan.

The Republican leader's campaign announced plans to seek a recount in Wisconsin and filed lawsuits to suspend the vote counts in several states.

Separately, Georgia also announced a recount of votes.

Ballot counting continues in a number of key battleground states where the margins are razor thin.

In a statement by his campaign, Trump said Biden is "rushing to falsely pose as the winner" of the US election, while Biden had not yet commented on the projection.

"The simple fact is this election is far from over. Joe Biden has not been certified as the winner of any states, let alone any of the highly contested states headed for mandatory recounts, or states where our campaign has valid and legitimate legal challenges that could determine the ultimate victor," the statement said.

In US elections, it is common practice for news organisations, after determining that a candidate no longer has a path to victory, to project a winner in the race.

The Trump campaign has said they will challenge the results in several states. Biden flipped battlegrounds Pennsylvania, Michigan, Wisconsin, and Arizona, all states Trump won in 2016, according to the Associated Press.

"Beginning Monday, our campaign will start prosecuting our case in court to ensure election laws are fully upheld and the rightful winner is seated," the statement by by Trump's campaign wrote. "The American People are entitled to an honest election: that means counting all legal ballots, and not counting any illegal ballots."

With Biden's projected win, Kamala Harris will be heading to the White House as his vice-president, and is set to create new records along the way.

Harris has had a trailblazing career, and will now be the first African-American, first woman and first person of Asian descent to serve as vice president.