US Attorney General Merrick Garland appointed a special counsel on Thursday to investigate President Joe Biden's handling of classified documents when he was vice-president during the administration of Barack Obama.

Biden acknowledged on Thursday that aides found a classified document in his "personal library" at his home in Wilmington, Delaware, and other documents in his garage, prompting the attorney general to name Robert Hur, a former Donald Trump-appointed US attorney in Maryland, to investigate the matter.

On Thursday, Biden reiterated that he and his attorneys are fully cooperating. The president said he will speak about the issue more,"God willing", soon.

Congressional Republicans had called on Garland to name a special counsel, just as he did over the classified documents found at former president Trump's Mar-a-Lago residence in Florida.

Information about the latest discovery came a few days after White House lawyer Richard Sauber revealed that Biden's personal lawyers discovered a batch of 10 classified documents in a "locked closet" on Nov 2, while cleaning out an office formerly used by the president at the Penn Biden Center for Diplomacy and Global Engagement in Washington. The discovery was only made public on Monday.

"The extraordinary circumstances here require the appointment of a special counsel for this matter," Garland said in appointing Hur.

Hur will take over from John Lausch, the top Justice Department prosecutor in Chicago, also a Trump-appointed attorney.

White House press secretary Karine Jean-Pierre said the White House found out about Garland's special counsel appointment only when it was announced.

Following Hur's appointment, Sauber said: "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."

Garland said the National Archives informed the Justice Department on Nov 4 about the first batch of classified documents. The FBI had assessed whether they had been mishandled on Nov 9.

Lausch was appointed to lead an investigation about the documents on Nov 14. And on Dec 20, the documents in the garage were found. Lausch then advised Garland on Jan 5 that appointing a special counsel was warranted.

'Irresponsible' mishandling

The first discovery prompted Biden's team to search other addresses. Garland said Biden's lawyers told the Justice Department about the discovery of more classified material at his home on Thursday.

Biden created the Penn Biden Center for Diplomacy and Global Engagement, which has headquarters at the University of Pennsylvania, and would use it occasionally from 2017 until he launched his 2020 presidential campaign in 2019.

The appointment of a special counsel to investigate Biden came shortly after Jack Smith was appointed special counsel for the criminal investigation into Trump, who had 300 documents at Mar-a-Lago.

Dozens of documents marked classified were found at Trump's Florida estate in August. The FBI carried out a raid only after attempts to recover them were rebuffed.

Biden lambasted Trump's mishandling of those highly classified documents as "irresponsible" in a September interview with CBS News' 60 Minutes.

Ohio Representative Mike Turner, the top Republican on the House Permanent Select Committee on Intelligence, urged Director of National Intelligence Avril Haines to conduct a review into the documents Biden had.

House Speaker Kevin McCarthy told reporters on Thursday: "I think Congress has to investigate this. They knew this has happened to President Biden before the election, but they kept it a secret from the American public? He goes on 60 Minutes, criticizes President Trump, even knowing what he has done, and he wasn't president at the time?"