Washington DC - A major package of air defenses for Ukraine has been announced by President Joe Biden as doubts over his political survival clouded the start of a NATO summit in Washington.

Leaders from the 32-nation military alliance were looking to put on a show of resolve against Russia during three days of pomp-filled ceremony in the US capital marking NATO's 75th anniversary.

But questions over Biden's fitness to govern threatened to dominate the gathering as the 81-year-old faces calls to quit the race for a second term after a disastrous debate performance against challenger Donald Trump.

Looking to refocus attention on US steadfastness, Biden kicked off the summit by announcing Washington will provide Ukraine with an additional Patriot air defense system.

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"The war will end with Ukraine remaining a free and independent country. Russia will not prevail," Biden said at a ceremony in the room where NATO's founding treaty was signed in 1949.

"This is a pivotal moment for Europe, for the transatlantic community, and, I might add, for the world," he said.

The US pledge comes on top of two new Patriot systems already being given by Germany and Romania, and one the Netherlands has said it is putting together with parts from other allies.

The air defenses "will help to protect Ukrainian cities, civilians, and soldiers," Biden said in a joint statement with other leaders, adding that they would look to send dozens more shorter-range systems in the coming months.

Ukraine has for months been clamoring for seven additional Patriot systems to help protect against devastating Russian strikes two and a half years into the invasion.

The war-torn country's vulnerability to Moscow's missiles was exposed by a strike Monday on a children's hospital in Kyiv.

NATO chief Jens Stoltenberg urged countries across the alliance to maintain their backing for Kyiv – warning that if Russia wins it will be the "greatest risk" for NATO.

"The outcome of this war will shape global security for decades to come," Stoltenberg said.

"The time to stand for freedom and democracy is now. The place is Ukraine."

As NATO sought to project unity and strength, doubts were swirling over the political future of the most powerful leader in the alliance.

Biden has so far defied pressure from some within his party to step aside after a calamitous TV debate against Trump last month fueled fears that he lacks mental acuity and physical fitness.

NATO members in Europe are nervously eying a potential return to the White House by Trump after November's election.

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On the campaign trail, the volatile ex-reality TV star has threatened to blow apart the principle of mutual self-defense that has underpinned NATO since it was founded in the wake of World War II.

In a post on his Truth Social network, Trump insisted that he had "made NATO viable again" by forcing European countries to spend more on their own defenses during his term in office.

"If it weren't for me as President, there probably would be no NATO by now," Trump claimed.

He said he now wants Washington's allies in Europe to pay up more to help Ukraine and ease the burden on the US.