Ukraine will stop Putin, forceful Biden tells NATO

US President Joe Biden has pledged to forcefully defend Ukraine against Russia's invasion at the NATO summit in Washington, using the global stage to show allies at home and abroad that he can still lead.

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 on June 27.

"(Vladimir) Putin wants nothing less, nothing less, than Ukraine's total subjugation ... and to wipe Ukraine off the map," Biden said on Tuesday in his welcome to NATO member states to the summit, referring to the Russian president.

"Ukraine can and will stop Putin. Today, NATO is stronger than it's ever been in its history."

Biden has rebuffed calls to step aside in his race against Republican Donald Trump, 78, vowing to beat him in November. So far, he has maintained the public support of most of his party's elite.

Biden closed his remarks by surprising NATO Secretary-General Jens Stoltenberg with the Presidential Medal of Freedom, clasping the highest US civilian award around the Norwegian politician's neck and crediting him with reviving the 32-member alliance.

The centrepiece of the NATO summit is set to be new commitments of military and humanitarian aid to Ukraine. US Secretary of State Antony Blinken said the summit would "further strengthen" the war-torn country's path to NATO membership.

Biden and the leaders of Germany, Italy, the Netherlands and Romania issued a joint statement with Ukrainian President Volodymyr Zelenskiy announcing the delivery of five additional Patriot and other strategic air defence systems to protect Ukrainian cities, civilians and soldiers.

They said additional strategic air defence systems would be announced this year.

Zelenskiy, who arrived in Washington on Tuesday and is due to meet with Biden on Thursday, has said Ukraine needs a minimum of seven Patriot systems, a goal met by the fresh deliveries announced on Tuesday.

"We are fighting for additional security guarantees for Ukraine - and these are weapons and finances, political support," he said on social media.

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."

Joe Biden
NATO leaders pose for a family photo at the Andrew W. Mellon Auditorium in Washington. (AP PHOTO)

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.

Biden will hold a rare solo press conference on Thursday, also aimed at quieting concerns.