US President Joe Biden has said he was surprised to learn that some classified documents were found at a private office he once used at a Washington think tank, asserting that he is unaware of what is in those papers and is "cooperating fully" with their review.

Speaking at a joint news conference with Mexico's President Lopez Obrador and Canadian Prime Minister Justin Trudeau, Biden said on Tuesday that he did not know that government records from his time as vice president had been taken to his think tank office.

Biden periodically used the office space from mid-2017, after his term as vice president for then-President Barack Obama ended, until the start of his 2020 presidential campaign.

“People know I take classified documents or classified information seriously. When my lawyers were clearing out my office at the University of Pennsylvania, they set up an office for me, a secure office in the Capitol. The four years after being the Vice President, I was a professor at Penn,” Biden told reporters in Mexico City.

The three leaders are in Mexico City to attend the 10th North American Leaders Summit.

A day earlier, the White House said that the Department of Justice was reviewing "a small number of documents with classified markings" from the previous Obama-Biden administration found at Biden’s think-tank office here.

“They found some documents in a box, in a locked cabinet, or at least the closet. And as soon as they did, they realised there were several classified documents in that box, and they did what they should have done. They immediately called the archives, turned them over to the archives,” Biden said in his first response to the discovery of documents.

“And, I was briefed about this discovery and surprised to learn that there were any government records that were taken there to that office, but I don't know what's in the documents either," he said.

The White House has convened a call with top allies to explain the investigation surrounding the classified documents, hoping to quell the growing criticism and questions about the discovery.

A White House official characterised the documents as “fewer than a dozen,” two people familiar with the call say, none of which are “particularly sensitive” and “not of high interest to the intelligence community.” "My lawyers have not suggested I ask what documents they were. I've turned over the boxes. They've turned over the boxes to the archives,” Biden said.

“We are cooperating fully with the review which, I hope, will be finished soon and will be more detailed at that time,” Biden said.

In August 2021, scores of classified documents were found at ex-president Donald Trump's residence at Mar-a-Lago in Florida during a raid by the FBI.

The documents were found more than a year after Trump departed the White House. He did not return them despite numerous requests by the National Archives and Records Administration (NARA).

Trump eventually gave 15 boxes of materials back to NARA in January last year. He is now under investigation for potentially mishandling classified documents.

The news about classified records turning up at Biden’s private office is sure to provide new fodder to Trump, who has already announced his 2024 presidential bid, according to CNN.

While the differences in the two incidents are stark, given Trump resisted turning over the documents he took, his team plans to highlight how these documents were found days before the midterm election but nothing was said publicly about it, it said.

Except for the headline, this story has not been edited by The Telegraph Online staff and has been published from a syndicated feed.