US President Joe Biden has admitted he "screwed up" in last week's debate against Donald Trump, but has vowed to fight on in the election race and moved to reassure key allies.

He told a Wisconsin radio station he made a "mistake" with his stumbling performance, but urged voters to instead judge him on his time in the White House.

On Wednesday, as reports suggested he was weighing his future, he worked to calm senior Democrats including state governors and campaign staff.

“I'm the nominee of the Democratic Party. No one's pushing me out. I'm not leaving," he said in a call to the broader campaign, a source told BBC News.

Mr Biden was joined on the call by Vice-President Kamala Harris, who reiterated her support.

Speculation has mounted over whether she could replace the president as the party’s candidate ahead of the November election.

A fundraising email sent after the call by the Biden-Harris campaign was also bullish. “Let me say this as clearly and simply as I can: I'm running,” Mr Biden said.

Questions have been swirling around whether the 81-year-old will continue with his campaign following a debate marked by verbal blanks and a weak voice.

It sparked concern in Democratic circles around his fitness for office and his ability to win the election.

Pressure on Mr Biden to drop out has only grown as more polls suggest his Republican rival’s lead has widened.

A New York Times poll conducted after the debate, which was published on Wednesday, suggested Trump was now holding his biggest lead yet at six points.

And a separate poll published by the BBC’s US partner CBS News suggested Trump has a three-point lead over Mr Biden in the crucial battleground states.

The damaging polling for Mr Biden has been compounded by some Democratic donors and lawmakers publicly calling on the president to stand aside.

Among them are Ramesh Kapur, an Indian-American industrialist based in Massachusetts, who has organised fundraisers for Democrats since 1988.

“I think it’s time for him to pass the torch,” Mr Kapur told the BBC. “I know he has the drive, but you can’t fight Mother Nature.”

And two Democrats in Congress also called for a change at the top of the party’s ticket. The second, Representative Raul Grijalva of Arizona, told the New York Times it was time for Democrats to “look elsewhere”.

Despite this, the White House and the Biden campaign have vehemently denied reports he is actively weighing his future and say he is committed to defeating Trump for a second time on 5 November.

The New York Times and CNN reported on Wednesday that Mr Biden had told an unnamed ally he was aware his re-election bid was in danger.

His forthcoming appearances - including an ABC News interview and a Friday rally in Wisconsin - were hugely important to his campaign, he reportedly said.