US Department of Justice (DoJ) investigators have found six more classified documents during a 13-hour search of President Joe Biden's home in Delaware, a lawyer for Mr Biden says.

Some documents seized at the Wilmington property on Friday were from his time as a senator and others from his tenure as vice-president under Barack Obama.

Lawyer Bob Bauer said "personally handwritten notes" and "surrounding materials" were also taken away.

Mr Biden and his wife were not present.

The president offered access "to his home to allow DoJ to conduct a search of the entire premises for potential vice-presidential records and potential classified material", Mr Bauer said in a statement on Saturday.

Former President Donald Trump is facing a criminal investigation for allegedly mishandling classified files.

Earlier this month Mr Biden's lawyers said a first batch of classified documents had been found on 2 November at the Penn Biden Center, a think-tank the president founded in Washington DC.

A second batch of records was found on 20 December in the garage at his Wilmington home, while another document was found in a storage space at the house on 12 January, his lawyers said.

After finding the documents, the president said his team immediately turned them over to the National Archives and the Justice Department. It is not clear why Mr Biden had kept them.

Under the Presidential Records Act, White House records are supposed to go to the National Archives once an administration ends, where they can be stored securely.

A special counsel, Robert Hur, has been appointed to lead the investigation into how the sensitive documents were handled.

The lengthy search and subsequent discovery of more documents is a political headache for the president, as he prepares to declare whether he will run for a second term in 2024.

Mr Biden and his wife, Jill, are spending the weekend in the coastal town of Rehoboth Beach in Delaware, where they own another house. It was searched earlier this month and no documents were found, his lawyers said, according to the New York Times.

The two-month gap between the first Biden discovery - days before the midterm elections - and the news being made public in January raises awkward questions for the president about transparency, BBC North America correspondent Anthony Zurcher says.

Mr Biden's team insists the president has co-operated fully with the DoJ inquiry. Mr Biden has played down the affair as an oversight, saying he has "no regrets" about not publicly disclosing the discovery of some classified files before the November midterm elections.

The discovery comes as former US President Donald Trump also faces a probe over his alleged mishandling of hundreds of classified documents at his Florida Mar-a-Lago residence and his alleged failure to comply with a subpoena.

Mr Trump and his lawyers resisted handing over the documents until the FBI raided his Florida holiday home last August. He alleges that President Biden is being treated more favourably by the FBI.

President Biden said at the time that Mr Trump's handling of the documents was "totally irresponsible".

Mr Trump has not given any reason to explain their presence there but has said he had the power as president to declassify them, a claim challenged by legal experts.