(AFP) - The US attorney general named an independent prosecutor Thursday to investigate President Joe Biden's handling of classified documents, after a furor over secret papers found at his former office exploded with the discovery of a second batch.

The new find -- like the original, from Biden's time as vice president -- was uncovered in the garage of his home in Wilmington, Delaware, where he often spends weekends, the White House said.

Attorney General Merrick Garland announced that Robert Hur, an appointee of Biden's predecessor Donald Trump, would be empowered to examine whether the cache violated any law.

"Under the regulations, the extraordinary circumstances here require the appointment of a special counsel," Garland, who runs the Justice Department, said.

The appointment came hours after the White House acknowledged the second batch of papers but did not address their contents -- supercharging a scandal over a first set found at a Washington think tank where Biden had an office.

Republicans in Congress, who have been accusing the White House of a cover-up over the affair, swiftly announced their own inquiry to run independently of the criminal probe.

The disclosures have prompted comparisons to the special counsel investigation of Trump's hoarding of hundreds of classified materials at his Florida beachfront home and his alleged obstruction of government efforts to get them back.

"I take classified documents and classified material seriously. We're cooperating fully (and) completely with the Justice Department's review," Biden told reporters.

"As part of that process, my lawyers reviewed other places where documents from my time as vice president were stored, and they finished the review last night."

The president said a "small number" of documents with classified markings had been found in storage areas and his library and that the Justice Department was notified immediately.

Biden declined to take shouted questions from the press, but a White House lawyer said later the documents had been "inadvertently misplaced."

Hur is a former assistant US attorney who worked in the DOJ from 2007 until 2014 and returned to public service under Trump as the principal associate deputy attorney general.

He promised a swift investigation and pledged to be "fair, impartial and dispassionate."

- 'Many questions' -

The first cache of Biden documents was discovered a week before last year's midterm elections but only acknowledged by the White House on Monday, prompting accusations from Republicans that it was kept secret for party political reasons.

James Comer, head of the powerful House oversight committee, said he would be leading an investigation into the "mishandling of classified documents and the Swamp's efforts to hide this information."

"There are many questions about why the Biden administration kept this matter a secret from the public, who had access to the office and the residence, and what information is contained in these classified documents," he said.

Trump, who is facing multiple criminal and civil probes, took to his Truth Social platform to call for an immediate end to his investigation "because I did everything right."

Legal analysts have pointed to major differences between the cases, particularly over the size of the huge hoard of documents Trump had stored at his residence after leaving the White House in 2021.

The FBI carted away some 11,000 papers after serving a search warrant in August, and Trump could face obstruction of justice charges after spending months resisting efforts to recover his trove.

The White House, in contrast, says it has been cooperating with the National Archives and the Department of Justice.

After the first batch of Biden documents was discovered at his former office at the Penn Biden Center think tank last November, lawyers turned them over to the National Archives, which handles all such materials, the White House counsel's office said.

Lawyers for Biden then scoured possible locations for any other stray documents.

Tempers flared during an exchange between Biden's press secretary Karine Jean-Pierre and reporters seeking to know if she was aware of any discussions in the White House about delaying public disclosure of the original discovery.

Jean-Pierre refused to answer, prompting a CBS News reporter to shoot back that Biden had pledged early in his presidency to acknowledge and correct mistakes.

"We don't need to have this kind of confrontation. Ask your question and I will answer them the best that I can," Jean-Pierre said.

© Agence France-Presse