This follows searches of the homes of President Joe Biden and former president Donald Trump that also turned up classified documents.

It wasn't immediately clear what information the documents contained or the level of classification they had been allocated.

This is the latest in a series of revelations about the apparently improper storage of papers dating from Biden's time as Barack Obama's vice-president.

This comes after a furor over secret papers found at his former office exploded with the discovery of a second batch.

As soon as the first batch of documents was discovered in Biden's former office last November, lawyers turned them over to the National Archives, which handles all such materials, the White House counsel's office says.