US President Joe Biden has claimed to be “the first black woman" to serve "with a black president” during a concerning radio interview which critics contend could be the final nail in the coffin of his 2024 campaign.

The president's latest flub adds to growing concerns about the extent of his cognitive decline.

The comment came days after Biden’s poor debate performance against Donald Trump.

Speaking on Philadelphia's WURD on Thursday, local time, Biden appeared to confuse himself with Vice President Kamala Harris.

The 81yearold said he was "proud to be the ... the first vice president, first black woman", adding to the confusion.

The oldest serving US president also referred to himself as the “first president that got elected statewide in the state of Delaware, when I was a kid".