Biden, 81, has endured 12 days of withering questions about his fitness for office as some of his fellow Democrats on Capitol Hill and campaign donors fear that he will lose the November 5 election after a halting debate performance in Atlanta on June 27.
"(Vladimir) Putin wants nothing less, nothing less, than Ukraine's total subjugation ... and to wipe Ukraine off the map," Biden said, referring to the Russian president. "Ukraine can and will stop Putin."
The White House is hoping he can turn the page on speculation with his speech, in which he spoke with a strong and confident voice and avoided any verbal flubs or signs of confusion that marked his debate performance.
Biden has rebuffed calls to step aside in his race against Republican Donald Trump, 78, vowing to beat him in November. So far, he still has the support of most of his party's elite.
The US president has made restoring traditional US alliances abroad the centrepiece of his foreign policy after Trump challenged allies as part of an "America First" approach. The winner in November could have a substantial impact on the future of NATO, Europe and the rest of the world.
Trump has suggested that, given a second term, he would not defend NATO members if they came under military attack and did not meet the alliance's defence spending target of 2% of their respective gross domestic product (GDP). He has also questioned the amount of aid given to Ukraine in its battle against Russia's invasion.
The centrepiece of the NATO summit was set to be new commitments of military and humanitarian aid to Ukraine, as well as a bridge for that war-torn country to join the 32-member alliance.
ZELENSKIY FIGHTS FOR MORE
Ukrainian President Volodymyr Zelenskiy arrived in Washington on Tuesday and said he would "fight" for NATO to strengthen Ukrainian air defences and furnish it with more F-16 fighter jets, requests that Washington appeared poised to grant.
"We are fighting for additional security guarantees for Ukraine - and these are weapons and finances, political support," he said in a video on social media.
Biden's national security adviser, Jake Sullivan, said NATO would announce a new military command in Germany for training and equipping Ukrainian troops and appoint a senior representative in Kyiv to deepen ties. US Secretary of State Antony Blinken said the summit would "further strengthen" Ukraine's path to NATO membership.
NATO Secretary-General Jens Stoltenberg told defence industry leaders in Washington that the leaders would pledge to enable arms makers across Europe and North America to produce more.
He also said NATO has placed an order for Stinger anti-aircraft missiles worth almost $US700 million ($NZ1.14 billion) in the name of several member states.
The shoulder-fired Stinger missiles have been in hot demand in Ukraine, where they have successfully stopped Russian assaults from the air, and in neighbouring European countries which fear they may also need to beat back Russian forces.
Zelenskiy is due to meet with Biden at the White House on Thursday and is scheduled to deliver an address on Tuesday evening (local time). After rebuffing calls from some fellow Democrats for Biden to step down, the White House hopes to turn the focus back to his ability to govern normally.
Aides said his opening speech at NATO would highlight what his administration sees as a key accomplishment: a stronger and more united NATO, under Washington's leadership, with more members and a resolve to meet their collective security needs.
That brings, they say, tangible results for American voters: a safer country, with a strong international economic position, more alliances and power abroad, and less at risk of conflict with its adversaries.
Trump and many Republican allies reject such arguments.
BIDEN'S STAYING POWER?
NATO, celebrating its 75th anniversary, has found new purpose in opposing Putin's Ukraine invasion and the grinding war will dominate private conversations between the leaders of the countries.
Those leaders, already anxious about the prospect of Trump's return, came to Washington with fresh concern about Biden's staying power, according to diplomats from their countries. One described Biden as bruised after a difficult political period and said their government was looking for signs about whether he would survive politically.
Biden will hold a rare solo press conference on Thursday, also aimed at quieting concerns.
NATO leaders face political uncertainty in Europe, with paralysis looming in France after gains for left and far right parties and German Chancellor Olaf Scholz's coalition weakened after a poor showing in European Parliament elections.
New British Prime Minister Sir Keir Starmer said as he headed to his first NATO summit that he would fulfil a campaign commitment to increase UK defence spending to 2.5% of GDP, but underlined he would only do so when the country could afford it and after a review of defence strategy.
Ukraine ultimately wants to join NATO to ward against further future attacks by Russia but candidates have to be approved by all of the alliance's members, some of which are wary of provoking a direct conflict with Russia.
Some members want the alliance to make clear Ukraine is moving toward NATO "irreversibly" and are keen for language in a summit statement beyond the alliance's pledge last year that "Ukraine's future is in NATO."
A senior NATO official said on Tuesday that Russia lacks the munitions and troops to start a major offensive in Ukraine and needs to secure significant ammunition supplies from other countries beyond what it already has.
But he estimated Russia would be able to sustain its war economy for three to four more years and also said "it will be some time" before Ukraine has amassed the munitions and personnel it needs to mount its own large-scale offensive operations.
