Washington DC - Joe Biden is not senile and has been on "good form" at the NATO summit, British Prime Minister Keir Starmer has said, as concerns grow about the US president's mental fitness.

Starmer defended Biden's leadership credentials amid questions about his cognitive health, which have intensified since criticism of his performance in a TV debate against Donald Trump.

The president (81) is seeking re-election in November despite discontent within the Democrat ranks about his suitability for a second term in office.

Starmer, who met the president for their first bilateral talks at the White House on Wednesday as NATO leaders gathered in Washington to discuss the challenges facing the bloc, said Biden had been "across all the detail."

Donald Trump Trump levels up threats against Liz Cheney with calls for new investigation

Starmer told broadcasters their one-to-one discussions had happened “at pace,” and Biden seemed "on really good form."

"We were billed for 45 minutes; we went on for the best part of an hour," he said. "He was absolutely across all the detail. We were going at pace through a number of issues."

Asked whether the president was senile, Starmer told the BBC: "No... he's shown incredible leadership.

"If there's one thing that came out of the council session yesterday, it was a clear understanding by everybody here that we're faced with more threats now than we've faced for many years and that we need the resolve of NATO.

"President Biden has led through some of the most challenging issues, actually, that we're facing globally."

The president on Monday stood firm against suggestions he should drop his candidacy and appealed for an end to the row over how the party moves forward in a sharply worded letter to Democrats.

Legislators and fundraisers, including Hollywood actor George Clooney, have since joined calls for Biden to step aside after he appeared flustered and stumbled over his words in the presidential debate with Trump last month.