The classified documents found at Joe Biden's former office and private residence were "inadvertently misplaced," a White House lawyer said Thursday, insisting the US president would "cooperate" with a special counsel appointed to investigate the matter.

"We are confident that a thorough review will show that these documents were inadvertently misplaced, and the President and his lawyers acted promptly upon discovery of this mistake," Richard Sauber, senior advisor to the White House Counsel's Office, said in a statement.