US President Joe Biden has welcomed Nato leaders to Washington DC with a forceful speech that seemed pitched to reassure allies overseas and closer to home that he can fight off an election challenge from Donald Trump.

In brief but strongly delivered remarks at the opening of the summit, the president declared the military alliance "more powerful than ever" as it faced a "pivotal moment" with the war between Russia and Ukraine.

Warning that "autocrats" had overturned global order", Mr Biden announced more military aid for Kyiv.

The US president and the leaders of Germany, Italy, the Netherlands and Romania are donating Patriot missile batteries and other systems to bolster Ukraine's beleaguered air defences.

"The war will end with Ukraine remaining a free and independent country," Mr Biden said in remarks read from a teleprompter on Tuesday afternoon. "Russia will not prevail. Ukraine will prevail."

Mr Biden spoke for about 13 minutes in a clear voice, a marked difference from his fumbling tone during last month's presidential debate with Trump.

Meanwhile, congressional Democrats met privately to debate Mr Biden's leadership of the party and the mood was "sad", lawmakers told the Associated Press news agency.

Later on Tuesday, a seventh House of Representatives Democrat - Mikie Sherrill of New Jersey - publicly called on Mr Biden not to run for re-election, saying the stakes were "too high".

Visiting diplomats also expressed scepticism about Mr Biden's future, according to reports.

"We don't see how he can come back after the debate," one unnamed European envoy told Reuters news agency. "I can't imagine him being at helm of the US and Nato for four more years."

Mr Biden's team has responded by trying to show that the 81-year-old remains vigorous enough to handle the demands of the presidency.

The White House has credited Mr Biden's leadership for the expansion of Nato since Russia invaded Ukraine two years ago, with Finland and Sweden joining the alliance.

Leaders from the 32 member countries are converging on the US capital for the summit.

Joining them is the UK's new Prime Minister Sir Keir Starmer. Before leaving for Washington, he said he was pleased to "confirm and reaffirm Labour's strong support, unshakeable support for Nato".

Asked by reporters travelling with him to the summit for his message to Russian President Vladimir Putin, Sir Keir said the gathering “should be seen as a clear and united resolve by Nato allies… to stand with Ukraine and stand up to Russian aggression”.

He said a deadly attack on a children’s hospital in Kyiv earlier this week “strengthens the resolve and that is a very important if tragic backdrop to this summit”.

Sir Keir added the package of support for Ukraine the UK was seeking to advance at the summit “goes beyond the support that's been put in before".

Sir Keir is due to meet Mr Biden on Wednesday, as well as Democratic and Republican leaders in Congress.

Tuesday's event was steeped in the alliance's history.

It was held in the very venue where the original treaty was signed decades ago, which Mr Biden invoked in his speech.

Near the conclusion of his speech, Mr Biden called Nato Secretary General Jens Stoltenberg on to the stage to award him the Presidential Medal of Freedom, America's highest civilian honour.

On Tuesday evening, Trump, 78, criticised Nato allies during a campaign rally at his golf club in Doral, Florida.

The event was attended by all three of his sons, including his youngest, Barron, who was at his first ever Trump rally, according to his father, and received more applause from the audience than his brothers, Eric and Don.

Trump has repeatedly lambasted Nato member states for failing to meet an agreed goal of spending at least 2% of gross domestic product on defence. Many more Nato countries now meet that benchmark since Russia invaded Ukraine.

Trump told supporters that these countries were contributing “hundreds of billions of dollars” more because when he was president he had warned them “No, I will not protect you from Russia” unless they paid more to the alliance.

When Trump made similar remarks at a rally in February this year, Mr Stoltenberg said such talk "undermines all of our security".

But the Nato chief remained tight-lipped on the Republican candidate as the summit began on Tuesday.

Ukrainian President Vlodymyr Zelensky said at an event held elsewhere in Washington DC that the whole world was "waiting for November", when the US general election takes place, and he urged American voters to stand by Ukraine.

Mr Zelensky is due to meet Mr Biden on Thursday.