Washington DC - President Joe Biden has been questioned as part of an investigation into the handling of classified documents found at his home and former private office, the White House said Monday.

The 80-year-old Democrat voluntarily gave the interview on Sunday and Monday, at a time when the president was also dealing with the fallout of a deadly attack on Israel by the Palestinian militant group Hamas and heavy Israeli airstrikes targeting Gaza.

The US attorney general appointed a special counsel in January to look into the handling of the secret files, which dated from Biden's time as vice president under president Barack Obama and immediately afterwards.

"The president has been interviewed as part of the investigation being led by Special Counsel Robert Hur," White House Counsel’s Office spokesperson Ian Sams said in a statement. "The voluntary interview was conducted at the White House over two days, Sunday and Monday, and concluded Monday."

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Sams added: "As we have said from the beginning, the president and the White House are cooperating with this investigation."

He referred further questions to the Justice Department.

The fact that Biden himself had been interviewed showed that the investigation was likely nearing its end. Special Counsel Hur's team had previously carried out extensive interviews among Biden's staff, ABC news said.

The probe involves documents found in the possession of Biden, who was vice president under Obama when the papers were removed from the White House.

Records were first unearthed in a private think tank office, where Biden used to work in Washington after his time as vice president, in November 2022.

More documents were found in the president's Wilmington, Delaware garage – next to his Corvette sports car – on December 20. Another set of files was then discovered in his home library on January 12.

Back in January, Biden dismissed the probe, saying there was "nothing there."

But mushrooming scandals over the handling of secret files have become an unexpectedly big issue in the 2024 US presidential elections.

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Former president Donald Trump, Biden's likely Republican opponent next year, separately faces trial over the alleged mishandling of classified documents after leaving office.

The special counsel in that case, Jack Smith, says Trump allegedly took classified documents to his Mar-A-Lago estate in Florida and refused to return them.

Trump (77) pleaded not guilty in June to charges of unlawfully retaining national defense information, conspiracy to obstruct justice, and making false statements.

The prosecutor in that case has asked for a 2024 trial, one of a number that Trump faces on various charges which include trying to overturn the results of the 2020 election, which he lost to Biden.

And Trump's vice president, Mike Pence, revealed in January that he too had uncovered documents marked as classified at his home.

Republicans have been trying to draw parallels between the Trump and Biden document cases, crying foul over what they say is special treatment meted out to the current president.

Biden and his family, meanwhile, face a host of their own legal issues, which threaten to add to concerns about the Democrat's low approval ratings and his age.

Republicans have launched an impeachment investigation over his son Hunter's business dealings with Ukraine and China, claiming without having shown any evidence so far that Biden senior benefited from them.

Hunter Biden also faces yet another special counsel probe into tax fraud and gun possession.