NATO allies on Wednesday announced they had started transferring F-16 jets to Ukraine while stepping up promises to Kyiv on eventual membership in the alliance, at a 75th anniversary summit clouded by political uncertainties in the United States.

With the pomp of the three-day gathering in the US capital, President Joe Biden is aiming to rally the West and also reassure voters amid pre-election scrutiny of whether at 81 -- six years older than NATO itself -- he remains fit for the job.

Biden individually welcomed the other 31 leaders of the alliance before urging them to keep pace with Russia's military production, which has stepped up sharply in the two years since President Vladimir Putin invaded Ukraine.

"We can -- and will -- defend every inch of NATO territory and we'll do it together," Biden told the North Atlantic Council, the formal decision-making body of the alliance, at Washington's convention center as the city sweltered under a heat wave.

Biden announced that Denmark and the Netherlands had begun sending US-made F-16 jets to Ukraine -- making good on a key promise last year to Kyiv, which has struggled to gain parity in the air with Russia.

He earlier announced new air defense systems for Ukraine and said the United States had agreed to place long-range missiles periodically in Germany.

In the evening Biden hosted the NATO leaders for a gala dinner, marked by storm clouds that forced the cancellation of a planned flypast.

Biden compared the alliance to his childhood neighborhood, saying: "When a neighbor needed help, you pitched in. When the bullies threatened the block, you stepped up."

US Secretary of State Antony Blinken said the F-16 transfer "concentrates Vladimir Putin's mind on the fact that he will not outlast Ukraine, he will not outlast us."

But White House challenger Donald Trump, who is edging out Biden in polls leading up to November's presidential election, has mused about bringing a quick peace settlement by pushing Ukraine to surrender territory to Russia.

The Republican mogul has repeatedly questioned the utility of NATO -- formed in 1949 as collective defense against Moscow -- which he sees as an unfair burden on the United States.

- 'Terror must fail' -

On the eve of the summit, Russia fired a barrage of missiles on Ukraine, killing dozens, including in Kyiv where a children's hospital was reduced to debris.

Biden invited Ukrainian President Volodymyr Zelensky to the summit, who voiced gratitude for the F-16s.

The new aircraft will "bring just and lasting peace closer, demonstrating that terror must fail," Zelensky wrote on social media.

The summit aimed in part to "Trump-proof" the alliance including by giving NATO a greater role, rather than the United States, in coordinating arms delivery into Ukraine.

In a joint declaration, NATO leaders promised to give Ukraine 40 billion euros ($43 billion) in military aid "within the next year" -- part of efforts to increase stability after Trump's allies in Congress held up US assistance for months.

Trump's aides have also discussed conditioning aid to Ukraine on forcing Kyiv to the negotiating table and said that China, not Russia, is a larger concern to US interests.

The NATO leaders' statement took aim at China as well, voicing "profound concern" over its industrial support to Russia.

China's reaction was swift.

"NATO should stop hyping up the so-called China threat and provoking confrontation and rivalry, and do more to contribute to world peace and stability," a spokesman for Beijing's mission to the EU said, adding that "China's position on Ukraine is open and aboveboard."

Biden invited four key Pacific partners to the summit -- Japan, South Korea, Australia and New Zealand -- as he seeks to increase NATO's role in Asia.

- 'Irreversible' Ukraine path to NATO -

The summit also stepped up promises to Ukraine, saying that it was on an "irreversible path to full Euro-Atlantic integration, including NATO membership."

Ukraine has for years sought but failed to win membership in NATO, which as an alliance considers an attack on one an attack on all.

But Biden and German Chancellor Olaf Scholz have led concerns that bringing in Kyiv now would effectively be entering war with nuclear-armed Russia as it occupies swathes of Ukrainian territory.

UK Prime Minister Keir Starmer, visiting days after his Labour Party swept to power, promised Zelensky that Britain -- unlike the United States -- was united across partisan lines on supporting Ukraine.

Starmer made clear he had no issue with Ukraine using UK missiles to strike into Russian territory, remarks that drew a rebuke from Moscow.

The summit, Starmer told reporters, is showing Putin that NATO is "bigger now than it's ever been, more united than it's ever been, and absolutely clear-eyed about the threat of Russian aggression."

© Agence France-Presse