Classified files recovered from US President Joe Biden's former private office reportedly include some material marked top secret.

Papers with the highest classification level were among about 10 files at a think tank bearing his name, according to the BBC's US partner CBS News.

Fewer than 10 classified documents were found at one of his Delaware homes, but none were top secret, says CBS.

A leak of top secret information could cause "exceptionally grave damage".

There are three basic levels of US classification: confidential, secret and top secret.

In total, roughly 20 classified files were recovered between the two locations, reports CBS, citing a federal law enforcement official familiar with the investigation.

The papers only began to come to light in news reports last Monday, but the first batch was found at Mr Biden's former institute, the Penn Biden Center in Washington DC, back in November.

That was days before the midterm elections, which saw Republicans narrowly win control of the US House of Representatives.

On Thursday it emerged that a second cache of files had been found on 20 December by Biden aides in a garage and adjacent room at his private home in Wilmington, Delaware.

On Friday, as he hosted Japanese Prime Minister Fumio Kishida at the White House, Mr Biden ignored questions from journalists about the growing controversy.

A day earlier, Mr Biden - who has previously described his predecessor Donald Trump's alleged mishandling of classified material as "totally irresponsible" - told reporters the documents were found locked in a garage next to his 1960s Chevrolet Corvette sports car.

The White House has said the files were "inadvertently misplaced" and that it is co-operating fully.

On Friday, House Republicans launched their own investigation, a day after Attorney General Merrick Garland appointed a special counsel to investigate the matter.

In a letter to the Department of Justice announcing the inquiry, Republican Jim Jordan, head of the House Judiciary Committee, questioned whether the department had "actively concealed this information from the public on the eve of the 2022 election".

"The American people deserve transparency and accountability from our most-senior executive branch law enforcement officials," the letter reads.

It calls for all documents related to the disclosures and the appointment of a special counsel to be provided no later than 27 January.

Earlier on Friday, House Oversight Committee Chairman James Comer wrote to the White House asking whether the president's scandal-plagued son, Hunter Biden, had access to files at the president's Wilmington residence.

Republicans are also calling on the White House to release visitor logs of Mr Biden's homes, but the White House has refused to say if such information will be divulged.

During his time in office, Mr Biden has spent nearly 200 days - or more than a fourth of his presidency - in his home state of Delaware, according to an Associated Press tally, including a stay in Wilmington this weekend.

Mr Trump is also under investigation by the justice department after he kept more than 300 classified files - including some marked with secret and top secret designations - at his Mar-a-Lago estate in Florida, some of which were seized by FBI agents executing a search warrant last August.