US President Donald Trump is heard on tape pleading with Georgia's election chief to overturn Joe Biden's win in the state, suggesting in a telephone call that the official "find" enough votes to hand Mr Trump the victory.

The phone call with secretary of state Brad Raffensperger on Saturday was the latest step in an unprecedented effort by a sitting American president to reverse the outcome of a free and fair election that he lost.

The president, who has refused to accept his loss to President-elect Mr Biden, repeatedly argued that Mr Raffensperger could change the certified results.

"All I want to do is this. I just want to find 11,780 votes, which is one more than we have," Mr Trump said. "Because we won the state."

Georgia counted its votes three times before certifying Mr Biden's win by an 11,779 margin. Mr Raffensperger noted: "President Trump, we've had several lawsuits, and we've had to respond in court to the lawsuits and the contentions. We don't agree that you have won."

Audio snippets of the conversation were first posted online by The Washington Post. The Associated Press obtained the full audio of Mr Trump's conversation with Georgia officials from a person on the call.

Mr Trump's renewed intervention and the persistent and unfounded claims of fraud come nearly two weeks before he leaves office and two days before twin run-off elections in Georgia that will determine political control of the US Senate.

The president used the hour=long conversation to tick through a list of claims about the election in Georgia, including that hundreds of thousands of ballots mysteriously appeared in Fulton County, which includes Atlanta. Officials have said there is no evidence of that happening.

The Georgia officials on the call are heard repeatedly pushing back against the president's assertions, telling him that he's relying on debunked theories and, in one case, selectively edited video.

At another point in the conversation, Mr Trump appeared to threaten Mr Raffensperger and Ryan Germany, the secretary of state's legal counsel, by suggesting both could be criminally liable if they failed to find that thousands of ballots in Fulton County had been illegally destroyed. There is no evidence to support Mr Trump's claim.

"That's a criminal offense," Mr Trump says. "And you can't let that happen."

Others on the call included Mark Meadows, the White House chief of staff, and lawyers assisting Mr Trump, including Washington lawyer Cleta Mitchell.

Democrats and a few Republicans condemned Mr Trump's actions, while at least one Democrat urged a criminal investigation. Legal experts said Mr Trump's behaviour raised questions about possible election law violations.

Biden senior adviser Bob Bauer called the recording "irrefutable proof" of Mr Trump pressuring and threatening an official in his own party to "rescind a state's lawful, certified vote count and fabricate another in its place".

"It captures the whole, disgraceful story about Donald Trump's assault on American democracy," Mr Bauer said.

Senator Dick Durbin of Illinois, the number two Democrat in that chamber, said Mr Trump's conduct "merits nothing less than a criminal investigation".

Mr Trump confirmed in a tweet on Sunday that he had spoken with Mr Raffensperger. The White House referred questions to Mr Trump's re-election campaign, which did not respond on Sunday to an emailed request for comment. Mr Raffensperger's office did not respond to a request for comment.

Mr Trump has repeatedly attacked how Mr Raffensperger conducted Georgia's elections, claiming without evidence that the state's 16 electoral votes were wrongly given to Mr Biden.

"He has no clue!" Mr Trump tweeted of Mr Raffensperger, saying the state official "was unwilling, or unable" to answer questions.

Mr Raffensperger's Twitter response: "Respectfully, President Trump: What you're saying is not true. The truth will come out."

Various election officials across the country and Mr Trump's former attorney general, William Barr, have said there was no widespread fraud in the election. Republican governors in Arizona and Georgia, key battleground states crucial to Mr Biden's victory, have also vouched for the integrity of their state elections.

Nearly all the legal challenges from Mr Trump and his allies have been dismissed by judges, including two thrown out the Supreme Court, which includes three Trump-nominated justices.

In Georgia, the ballots were counted three times, including a mandatory hand count and a Trump-requested recount.

Still, Mr Trump has publicly disparaged the election, worrying Republicans that may discourage party voters from participating in Tuesday's run-offs pitting Senator Kelly Loeffler against Democrat Raphael Warnock and Republican David Perdue against Democrat Jon Ossoff.

Rebecca Green, who helps direct the election law programme at William and Mary Law School, said that while it is appropriate for a candidate to question the outcome of an election, the processes for doing so for the presidential election have run their course. States have certified their votes.

Ms Green said Mr Trump had raised "lots of questions" about whether he violated any election laws.

Carl Tobias, a law professor at the University of Richmond, said Mr Trump is guilty of "reprehensible and, possibly illegal, conduct".

Mr Trump noted on the call that he intended to repeat his claims about fraud at a Monday night rally in Dalton, a heavily Republican area in north Georgia.

"The people of Georgia are angry, the people of the country are angry," he says on the recording.

Vice president-elect Kamala Harris criticised Mr Trump for the call.

"It was a bald, bald-faced, bold abuse of power by the president of the United States," she said.