US President Joe Biden says he was surprised to learn that classified documents were found in a think-tank office he once used and said he and his team are co-operating fully with a review of what happened.

President Biden told reporters at a joint news conference with the leaders of Mexico and Canada that he took classified documents seriously. He said he did not know what was in the documents.

“We are co-operating fully with the review, which I hope will be finished soon,” President Biden said on Tuesday.

A Democrat, President Biden has faced criticism from Republicans after his Justice Department launched an investigation last year into Republican former president Donald Trump’s handling of classified documents discovered at his Mar-a-Lago club in Florida.

The cases differ in several ways.

President Biden’s lawyers discovered fewer than a dozen classified records and informed the US National Archives of their discovery, turned over the materials, and said they were co-operating with the Archives and the Justice Department.

By contrast, Mr Trump kept thousands of government records, a few hundred of which were marked as classified, inside his Florida residence for more than a year after departing the White House, and did not return them immediately or willingly despite numerous requests by the National Archives.

When he finally handed over 15 boxes of records in January 2022, the Archives discovered more than 100 were marked as classified. It later referred the matter to the Justice Department.

Investigators tried to get Mr Trump to return any remaining classified records through a grand jury subpoena and a visit to Mr Trump’s Mar-a-Lago estate. On that visit, Mr Trump’s advisers returned a few dozen additional classified records, and attested that no other classified material remained in the residence.

Suspecting possible obstruction of justice, the FBI sought and obtained court approval in August to search his home, where agents found more than 13,000 additional records, about 100 of them highly classified.

US Attorney-General Merrick Garland appointed Special Counsel Jack Smith in November to investigate the matter.

President Biden defended the handling of the documents.

“They found some documents in a box in a locked cabinet, or at least a 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,” he said.

“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.”

– AAP