The United States and Europe must stick together, NATO chief Jens Stoltenberg and US President Joe Biden said on Thursday, as the Western military alliance turned 75 menaced by an aggressive Russia and the specter of Donald Trump's return to power.

The Kremlin's full-scale invasion of Ukraine in 2022 re-invigorated NATO as it was confronted by one of the most serious challenges since it emerged from the ashes of World War II to counter the Soviet Union and its Warsaw Pact group.

The alliance has bolstered its forces across eastern Europe and grown to 32 members after Finland and Sweden joined its ranks.

But while the war has refocused NATO's attention on its old nemesis Moscow to the east, there is also another threat unnerving allies from leading power the United States in the west.

That's the possible return to the White House of Trump, who undermined NATO's collective defense guarantee by saying he'd encourage Russia to attack any members not spending enough on defense.

"I do not believe in America alone, just as I don't believe in Europe alone," Stoltenberg said at a ceremony at NATO's Brussels headquarters.

"I believe in America and Europe together in NATO, because, fundamentally, we are stronger and safer together."

In a bid to stave off Trump's criticisms, NATO has showcased increased spending by its European allies — with 20 members this year set to hit a target of two percent of GDP for defence.

"North America also needs Europe," Stoltenberg said, after a Belgian military band played the NATO anthem.

"Through NATO, the United States has more friends and more allies than any other major power."

Biden — who will face off against Trump at elections in November — insisted that the United States must stick to the promises it has made to its allies in Europe to help protect them if attacked.

"We must remember that the sacred commitment we make to our allies — to defend every inch of NATO territory — makes us safer too," he said in a statement.

Ukraine wants air defense

While Trump looms over the future of the alliance, NATO countries face the more pressing challenge of ensuring Ukraine does not lose its fight to push back Russia.

Alliance members have thrown their weight behind Kyiv — which is bidding to join NATO — by sending Ukraine weapons worth tens of billions of dollars.

But those supplies have now dwindled as crucial US support remains blocked by political wrangling. On the frontline, Ukraine's outgunned forces have been pushed onto the back foot.

In the face of surging Russian missile attacks on its infrastructure, Kyiv is pleading with its Western backers to send all the Patriot defense systems they can spare.

"I don't want to spoil the party, but of course my main message today will be Patriots," Ukrainian Foreign Minister Dmytro Kuleba said before talks with his NATO counterparts.

"Saving Ukrainian lives, saving the Ukrainian economy, saving Ukrainian cities depends on the availability of Patriots and other air defense systems in Ukraine."

Stoltenberg said after the talks that NATO allies had agreed to "go back and look into their inventories" to see if they could spare any more air defense systems for Ukraine.

Any prospect of a potential defeat for Ukraine has sent shivers through NATO allies close to Russia who fear they could be next in the Kremlin's sights.

"Nice stories don't win wars," Lithuania's foreign minister Gabrielius Landsbergis wrote on X.

"Without significant deliveries of weapons and real security guarantees the glorious narrative of unity and solidarity with Ukraine is wearing thin and rapidly approaching cynicism."

In a bid to ensure long-term support for Kyiv in the face of a possible return by Trump, Stoltenberg has proposed NATO members set up a 100-billion-euro ($108-billion) five-year fund.

As part of the plan he is also pushing to get NATO as an organization more directly involved in coordinating deliveries.

That is something the alliance has so far refused to do out of concern it could drag it closer to war with Russia.

NATO countries gave the go-ahead to work on the plan Wednesday, but there still remain many questions over financing and how far the alliance is willing to go to thrash it out before a summit in Washington in July.