BRUSSELS, Apr. 4 (Reuters) - NATO alliance members agreed on Thursday to scour their arsenals for more air defence systems to protect Ukraine from Russian ballistic missile attacks, as the alliance marked a 75th anniversary overshadowed by the war on its borders.

"Allies understand the urgency," NATO Secretary General Jens Stoltenberg said after Ukrainian Foreign Minister Dmytro Kuleba met NATO counterparts and appealed for new additional air defence systems, especially U.S.-made Patriot missiles.

"Allies will now go back and look into their inventories, look into if there are any ways they can provide more systems, in particular Patriots, but also of course ensure that the systems which are already there have the ammunition but also the spare parts (they need)," Stoltenberg told a news conference.

U.S. Secretary of State Antony Blinken said more support for Ukraine was especially important as countries like China, North Korea and Iran were supporting Russia's efforts to build up its defence industrial base.

"I believe based on what I heard today that everyone, including the United States, is going to double back, and if necessary double down, on finding the resources that Ukraine continues to need," Blinken said at a news conference, citing the need for air defence, as well as for artillery and munitions.

Neither official spelled out any specific pledge or aid target.

Earlier, as a ceremony with birthday cake and marching bands could not mask the sombre mood in Brussels, Kuleba told reporters: "I didn't want to spoil the birthday party for NATO, but I felt compelled to deliver a very sobering message on behalf of Ukrainians about the state of Russian air attacks on my country."

Overnight Thursday, a Russian drone attack struck residential buildings in the Ukrainian city of Kharkiv and an energy facility in the region, killing several people and cutting power for 350,000 residents, Ukrainian officials said.

"NATO's biggest battles to fight are still in the future, and we have to be ready for them," Lithuanian Foreign Minister Gabrielius Landsbergis told reporters.

A NATO official said Russia was likely recruiting about 30,000 additional personnel per month and could absorb losses on the battlefield and continue attacks. Russia, however, was still lacking munitions and manoeuvre units required for a successful major offensive, the official said on condition of anonymity.

On Wednesday, the NATO ministers agreed to start planning for a greater role in coordinating military aid to Ukraine to help it battle Russia.

"This is an ongoing discussion that we’ll have in the weeks ahead, and I imagine that you'll see something at the summit when we come together in Washington in July," said Blinken.

But it was far from clear whether a 100-billion euro, five-year fund proposed by Stoltenberg would be accepted by NATO members, who take decisions by consensus. Hungary has signalled opposition and other members have warned against duplicating bilateral and European Union aid efforts for Ukraine.

European leaders are anxious about the alliance's future if Donald Trump beats incumbent President Joe Biden in the November U.S. election in November, but also about the multi-billion Ukraine aid package held up in the U.S. Congress.

"Europe needs North America for its security," Stoltenberg said.

"At the same time, North America also needs Europe. European allies provide world-class militaries, vast intelligence networks and unique diplomatic leverage, multiplying America's might."

NATO began with 12 members from North America and Europe, founded in response to growing fears that the Soviet Union posed a military threat to European democracies during the Cold War.

At its heart is the concept of collective defence and that an attack on one member is considered an attack on all, giving U.S. military protection to Western Europe.

It has retaken a central role in world affairs after Russia invaded Ukraine in February 2022, leading European governments to view Moscow once more as a major security threat.

NATO's two newest members, Finland and Sweden, joined in direct response to Russia's invasion of Ukraine.

In Moscow, the Kremlin's spokesman said Russia and NATO were now in "direct confrontation" due to the alliance's enlargement.

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