U.S. president-elect Joe Biden has been declared the winner in the state of Georgia - flipping the historically Republican-voting state for the Democratic Party for the first time in decades.

The Associated Press news agency said late Thursday that Biden had secured Georgia's 16 Electoral College votes - stretching his lead to 306 Electoral College votes compared with U.S. President Donald Trump's 232.

The news agency declared Biden the winner in the U.S. presidential elections after the former U.S. vice president won Pennsylvania, Michigan and Wisconsin - but the state of Georgia had been too close to call.

Georgia Secretary of State Brad Raffensperger's office announced a manual audit of the presidential contest results had confirmed Biden's narrow lead over Trump.

A hand tally of about 5 million votes was carried out under an audit required by a Georgia state law, The Associated Press reported - and not owing to any problems with election procedures or an official recount request.

The state has until Friday to certify results that have been certified and submitted by the counties, the news agency said.

The Trump campaign has filed several lawsuits in pivotal states across the U.S. as the Republican president has alleged widespread voter fraud marred the elections.

Trump has continued to refuse to concede the race, with his personal lawyer Rudy Giuliani saying Thursday that the campaign has "enough evidence" to "overturn" Biden's victory.

Raffensperger welcomed the results of the state's manual count Thursday evening, calling the process "historic."

"Georgia's historic first statewide audit reaffirmed that the state's new secure paper ballot voting system accurately counted and reported results," he said in a statement posted on his website.

"This is a credit to the hard work of our county and local elections officials who moved quickly to undertake and complete such a momentous task in a short period of time."

Raffensperger's office said that because Biden's margin of victory was less than 0.5% the Trump campaign can request a recount after the results are certified.

"That recount will be conducted by rescanning all paper ballots," the statement reads.

The Trump campaign did not immediately comment on The Associated Press's victory call for Biden or the audit results late Thursday.