Mr Biden's lawyer has said in statement that the discovery was made up of a "small number" of records and all-but-one were found in a storage space in the garage. The other was in a room next door.

The lawyer said that the Department of Justice was immediately notified and the documents handed over.

The statement added: "As we stated previously, we are fully cooperating with the National Archives and the Department of Justice in a process to ensure that any Obama-Biden Administration records are appropriately in possession of the Archives."

It comes after the White House said on Monday that the US Justice Department was reviewing a batch of potentially classified documents found in the Washington office space of the president's former institute.

Special counsel to the president, Richard Sauber, said "a small number of documents with classified markings" were discovered as Mr Biden's personal lawyers were clearing out the offices of the Penn Biden Center.

Mr Biden had kept an office there after he left the vice presidency in 2017 until shortly before he launched his 2020 presidential campaign in 2019.

The president's aides have since been searching for any additional classified materials that may be in other locations he used, according to the source, who spoke on the condition of anonymity to provide details about the ongoing inquiry.

The Justice Department has not yet commented.

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A source told NBC News that Mr Biden's aides have been sifting through documents stored at locations beyond his former Washington office to determine if there are any other classified documents that need to be turned over to the National Archives and reviewed by the Justice Department.

The search was described as exhaustive, with the goal of getting a full accounting of all classified documents that may have inadvertently been packed in boxes when Biden cleared out of the vice president's office space in January 2017.

The initial documents were found on 2 November last year in a "locked closet" in the office, Mr Sauber said.

He added that the lawyers immediately alerted the White House Counsel's Office, which notified the National Archives and Records Administration and subsequently took custody of the documents the next day.

"Since that discovery, the president's personal attorneys have cooperated with the Archives and the Department of Justice in a process to ensure that any Obama-Biden Administration records are appropriately in the possession of the Archives," Mr Sauber said.

On Tuesday, Mr Biden told reporters that he was "surprised" by the discovery, and that he didn't know what was in the documents.

For months, the Justice Department has been investigating the retention of roughly 300 classified documents that were recovered from the Florida estate of Donald Trump.

Prosecutors have interviewed an array of Mr Trump's associates and have been using a grand jury to hear evidence.

It remains unclear when a decision will be made on whether Mr Trump, or anyone else, should be charged over the papers.