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.

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 defence guarantee by saying he'd encourage Russia to attack any members not spending enough on defence.

"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."

- Ukraine wants air defence -

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 defence systems they can spare.

"At the same time we are celebrating ... Ukraine is having a difficult time," said Estonian Foreign Minister Margus Tsahkna.

"We must give rapid support to Ukraine. The military support, ammunition but also air defence."

Stoltenberg, meanwhile, has proposed a 100-billion-euro ($108-billion) five-year fund in a bid to ensure long-term support for Kyiv.

He is also pushing to get NATO as an organisation more directly involved in coordinating deliveries, something the alliance has so far refused to do out of concern it could drag it closer to war with Russia.

Part of the urgency for the plan, officials say, is to try to protect support for Ukraine from Trump's possible return.

But there remain many questions over how any financing would work and allies will look to thrash out details by a summit in Washington in July.