Several more senior members of President Joe Biden's Democratic party have privately joined calls for him to hand over his candidacy for November's election to another candidate, US media report.

During a group call with House Minority Leader Hakeem Jeffries, four congressmen were explicit in urging Mr Biden to step aside, according to outlets including the BBC's US partner CBS News.

The quartet were joined by others who voiced concerns about Mr Biden's fitness for office after a recent stumbling debate performance, but stopped short of asking for the president to clear the way, CBS added.

Mr Biden has vowed to fight on, and has retained the support of other Democrats who insist he is the man to beat Donald Trump in November's vote.

Scrutiny of Mr Biden's candidacy is expected to intensify on Monday with the return of lawmakers to Capitol Hill. The president will also be in the spotlight in the days ahead as he hosts a summit in Washington for leaders of Nato countries.

American media reports slightly diverged on which Democratic representatives said what during the private conversation with Mr Jeffries on Sunday.

Jerry Nadler, Mark Takano and Adam Smith all said Mr Biden should step aside, according to multiple outlets, citing people on the call or familiar with what was said.

Joe Morelle added his voice, according to CBS and the New York Times, but the Associated Press said that the fourth person was Jim Himes.

Others expressed concerns about Mr Biden's electoral chances against Trump. None of the representatives have made their comments public.

Long-standing questions about Mr Biden's age escalated after last month's televised head-to-head with Donald Trump, in which the president stumbled over some answers or appeared to lose his train of thought.

Various explanations were offered by the Biden camp, including that the president had a cold and was exhausted by travel.

A day afterwards, Mr Biden was defiant at a rally in Wisconsin, although this did little to quell the fears of some allies. Last week, Lloyd Doggett became the first Democrat in Congress to urge Mr Biden to step aside.

He was publicly echoed last week by at least four colleagues - and later, privately, by the additional four who reportedly gave their views to Mr Jeffries on Sunday.

Trump has celebrated Mr Biden's shaky showing and last week labelled his rival "broken-down". He is three years Mr Biden's junior as he bids to return to the White House, despite a recent conviction in a New York criminal hush-money case.

Amid mounting speculation over Mr Biden's candidacy in November, the thoughts of some Democrats have turned to who could replace him.

Some party members have rallied around Vice-President Kamala Harris, who currently occupies the position of Mr Biden's running mate in November.

Another representative, Adam Schiff, has said Ms Harris could beat Trump "overwhelmingly", although speaking to NBC News, he too stopped short of telling Mr Biden to drop out.

In an apparent signal that he was switching his crosshairs to Ms Harris, Trump has suggested the vice-president would be "better" than Mr Biden, but still "pathetic".

Mr Biden and his allies have spent days recently insisting that he is still up to the job of defeating Trump for a second election in a row - despite reportedly being aware that the coming days and weeks are critical to salvaging his campaign.

During a pair of interviews last week, Mr Biden acknowledged that he had "screwed up" but later vowed that only the "Lord Almighty" could convince him to end his bid to win the White House again.

Speaking to ABC News on Friday, the president declined to take a cognitive test and make the results public in order to reassure voters he was fit to serve another term.

"I have a cognitive test every single day," he said. "Every day I have that test - everything I do [is a test]."