A search by the US Justice Department of President Joe Biden’s home had been carried out after a “voluntary, proactive offer” by his personal lawyers to the department, the White House says.

The White House Counsel’s Office also said it was reviewing recent record requests from the Republican-led House oversight committee and pledged to respect legislative oversight but warned its co-operation might be limited by executive privilege and an ongoing Department of Justice investigation.

A new search of Biden’s home in Wilmington, Delaware, by the department on Friday had found six more items, including documents with classification markings, a lawyer for the president said.

It was the latest in a string of discoveries of classified documents at President Biden’s Wilmington home and at a temporary office at the University of Pennsylvania.

Some of the most recently disclosed classified documents and “surrounding materials” dated from President Biden’s tenure in the US Senate, where he represented Delaware from 1973 to 2009, according to his lawyer, Bob Bauer.

Other documents had been from his tenure as vice-president in the Obama administration, from 2009 through 2017, Mr Bauer said.

“This was a voluntary, proactive offer by the president’s personal lawyers to DOJ to have access to the home,” White House spokesperson Ian Sams said.

Mr Sams declined to provide more clarity on the exact content of the materials taken from the Wilmington house. President Biden had been kept informed throughout this process, the White House said.

The search increases the legal and political stakes for President Biden, who has insisted that the previous discovery of classified material at his home and former office would eventually be deemed inconsequential.

Mr Sams also said the White House counsel had sent a letter to James Comer, the chairman of the Republican-controlled House of Representatives oversight committee, in response to Comer’s inquiries about the classified documents found at Democrat Biden’s home and office.

The White House said it was seek to “accommodate legitimate oversight interests within the committee’s jurisdiction while also respecting the separation of powers and the constitutional and statutory obligations of the executive branch”.

A committee spokesperson said the White House’s suggestion that it needed to determine whether the requests were related to legitimate oversight undercut President Biden’s public pledge of transparency.