Joe Biden's campaign faced further pressure on Thursday amid reported concerns from Barack Obama about the presidential election, a darkening mood among Democrats and polls suggesting Donald Trump was pulling ahead.

Some Democrats painted a bleak picture. One senior party official told the BBC that many in the party felt Mr Biden's stepping down was "inevitable".

Polling on Thursday by the BBC's US partner, CBS News, showed him five points behind Trump - the widest margin recorded this campaign.

But Mr Biden's campaign batted away reports of high-level Democratic concern as "baseless", insisting he would remain the nominee.

Mr Obama was reported by the Washington Post to have privately stated Mr Biden's chances were greatly diminished. Spokespeople for the former president have declined to comment.

It followed several reports that former house speaker Nancy Pelosi and the two most senior Democrats in Congress, Hakeem Jeffries and Chuck Schumer, had advised Mr Biden to consider his candidacy for the good of the party. All have rejected the reports.

But a senior Democrat source told BBC News that the mood in Washington was grim, adding: "We are all waiting for the inevitable decision."

Adam Smith, a Democratic congressman for Washington state, painted a similarly grim picture. Asked by BBC Radio 4's The World Tonight whether the party was "coming to the end" of Mr Biden's candidacy, he said: "That is my sense".

"I mean, I don’t know. But without question, I think that is the direction that this is heading right now."

Mr Biden has faced a torrid few weeks since his poor showing in the first presidential debate late last month. He is currently in isolation in Delaware while he recovers from a Covid infection.

By contrast, Donald Trump officially accepted his party's presidential nomination at the Republican national convention in Milwaukee on Thursday evening.

He struck a confident tone in his first speech since surviving an assassination attempt. Delegates and supporters at the convention have been in high spirits all week.

Mr Biden has so far taken a defiant tone in response to Democratic pressure for him to step aside as his party's candidate. He continues to enjoy the public support of many politicians, including members of the powerful Congressional black caucus.

Mr Schumer, the Senate Majority Leader and Mr Jeffries, the ranking Democrat in the House of Representatives, are reported to have told Mr Biden last week that their colleagues in Congress were "concerned" that his troubles would hit their own chances of re-election.

Mr Schumer said the reports were "idle speculation", while Mr Jeffries said his was "a private conversation that will remain private".

CNN meanwhile reported that Mrs Pelosi had told Mr Biden that polls show he cannot win. She later slammed the reporting as a "feeding frenzy", but did not deny that a conversation with Mr Biden had taken place.

Jamie Raskin, a congressman from Maryland, wrote to Mr Biden, comparing him to a baseball pitcher at the end of his career - saying there was "no shame" in retiring "to the overflowing appreciation of the crowd when your arm is tired out".

But TJ Ducklo, a Biden campaign senior adviser, slammed reports of grandee concern as "baseless conjecture from anonymous sources".

"Joe Biden is his party's nominee," he wrote on X. "He's running for re-election."

Deputy campaign manager Quentin Fulks said Mr Biden was "not wavering on anything. The president has made his decision. I don't want to be rude, but I don’t know how many more times we can answer that."

Mr Biden has mild upper respiratory symptoms associated with Covid but does not have a fever, presidential doctor Kevin O'Connor said on Thursday,

The White House said he was expected to meet Israeli Prime Minister Benjamin Netanyahu after he arrives in the US on Wednesday.