NATO head Jens Stoltenberg said on Thursday alliance members "are not providing Ukraine with enough ammunition" and the shortfall has allowed Russia to push Kyiv's forces back.
"The Ukrainians are not running out of courage. They are running out of ammunition," Stoltenberg told journalists.
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"Together, we have the capacity to provide Ukraine what it needs. Now we need to show the political will to do so."
Stoltenberg's warning comes as military support from key Kyiv backer, the United States, remains blocked in Congress by hardline Republicans opposed to President Joe Biden.
"All allies need to dig deep and deliver quickly. Every day of delay has real consequences on the battlefield in Ukraine," the secretary general of the Western military alliance said.
"So this is a critical moment and it will be a grave historic mistake to allow Putin to prevail."
Ukrainian and Western officials have said that Kyiv's forces are being outgunned by Russia as support from key backers has dwindled.
Ukraine was recently forced out of the strategic stronghold of Avdiivka after a fierce battle and its soldiers are struggling to stop Russia pushing further.
While political deadlock is stalling support from Washington, European countries are struggling to ramp up production of ammunition and arms from their industry.
"We need the decisions to invest more in the defence industry," Stoltenberg said.
"We need to ensure that our governments are agreeing contracts with the defence industry so they can make the commercial decisions to scale up production."
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The European Union has launched a string of initiatives to try to increase output and insists that levels are picking up.
But the 27-nation bloc has fallen well short of a promise made last year to give Ukraine a million artillery shells by this month.
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