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 a looming 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 faces its greatest challenge in a generation with the war in Ukraine.

He called it a “pivotal moment” for Europe and the world.

Warning that “autocrats have overturned global order” and terrorist groups continued to pose a threat, Mr Biden announced increased military aide for Ukraine and celebrated the addition of Sweden and Norway to the alliance.

“The war will end with Ukraine remaining a free and independent country,” Mr Biden said. “Russia will not prevail. Ukraine will prevail.”

US officials said the air defence aid includes Patriot missiles and their components. In coming months, the allies also will provide Ukraine with dozens of other tactical air defence systems.

Mr Biden spoke for about 13 minutes on Tuesday afternoon in a clear voice, a marked difference from his fumbling and softly spoken tone during last month’s presidential debate.

His speech took place at a fraught moment in his political career. He faces calls to step aside as the Democratic Party’s presidential nominee following a disastrous performance at last month’s presidential debate.

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.

His administration highlighted his role hosting the Nato summit this week in Washington DC as the alliance celebrates its 75th anniversary.

Leaders from the 32 member countries, including the UK’s new Prime Minister Sir Keir Starmer, gathered on Tuesday evening for Mr Biden’s address.

Before leaving for Washington, Sir Keir said he was pleased to “confirm and reaffirm Labour’s strong support, unshakeable support for Nato”.

The Nato trip is “all about standing together with our allies, discussing practically how we provide further support to Ukraine, and send a very, very clear message to Putin that we will stand against Russian aggression wherever it is in the world,” he said.

The 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.

After the leaders posed for a group photograph, the audience watched a video celebrating Nato, featuring archival footage of Nato troops and rousing speeches from world leaders.

In opening remarks, Nato Secretary General Jens Stoltenberg celebrated the creation of a group “with a clear purpose to preserve peace and safeguard freedom”.

The group of nations currently faces a major challenge as Russia continues its war against Ukraine.

Mr Stoltenberg called it the “biggest security crisis in generations.”

Near the conclusion of his speech, Mr Biden called Mr Stoltenberg back to the stage, calling him “a man of integrity and intellectual rigour.”

He awarded Mr Stoltenberg the Presidential Medal of Freedom, America’s highest civilian honour.