Faced with growing American reluctance to send more military aid to Ukraine, European leaders are moving to fill the gap, vowing new support for Kyiv as it battles Russia in a war in Europe’s backyard.

Several countries — including Germany, Britain and Norway — are increasing production of weapons, especially the artillery ammunition that Ukraine so badly needs. Germany, once a laggard in providing aid to Ukraine, announced a week ago that it planned to double its support to $8.5 billion in 2024 and would deliver more crucial air-defense systems by the end of this year. And European Union states are gearing up to train an additional 10,000 Ukrainian soldiers, bringing the total so far to 40,000.

“We really have to step up our game here,” Kajsa Ollongren, the Dutch defense minister, said at a forum this month at the Clingendael Institute, a think tank funded by the Dutch government.

But that may be little comfort to Ukraine, where a counteroffensive against invading Russian forces has stalled as winter approaches, and officials say more support is needed now, even as many countries turn their attention to the Israel-Gaza war.

In a worrying sign, the E.U. appears likely to fail an early test of its ability to sustain backing for Ukraine. A much touted pledge to donate one million rounds of 155-millimeter-caliber shells within one year to Ukraine is now widely expected to fall short.

“The million will not be reached — we must assume it,” Germany’s defense minister, Boris Pistorius, said this week, acknowledging the bloc will miss the March 2024 deadline.

European officials have long worried that rising Republican opposition to the military support that the United States is sending to Ukraine — $45 billion in weapons and other equipment so far — would diminish the leading American role in funding the war should President Biden lose re-election.

Those concerns were made all the more acute this month when House Republicans shelved Mr. Biden’s $105 billion plan for emergency aid for several world crises, including about $61.4 billion for Ukraine.

Unless, or until, the budget standoff is resolved, officials in Washington and Kyiv are left to weigh how best to spend the remaining $4.9 billion in previously approved security assistance for Ukraine if that is the last available source of American funding for the foreseeable future.

“We Europeans, who have the necessary means to do so, have to be willing politically and materially to help Ukraine and to continue to do so, even to take over from the United States if, as is perhaps likely, its support diminishes,” Josep Borrell Fontelles, the European Union’s top diplomat, said recently.

Russia’s invasion of Ukraine in February 2022 jolted European leaders who realized their militaries and defense industries were ill-prepared for the war in their backyard. It was a “rude awakening,” Sweden’s defense minister, Pal Jonson, said at the Clingendael forum, but one that united most of Europe behind Ukraine — considered by many to be something of a buffer zone between Russia and NATO.

“If the West stops supporting Ukraine, there will be no more Ukraine and no more European security architecture,” Yonatan Vseviyov, a top Estonian diplomat, said in an interview published on Friday on the Ukrainian news agency RBC.

Some European countries are already responding.

Although there is not unanimous support for Ukraine — Slovakia has said it will stop military aid to Kyiv, and Hungary is trying to stall new E.U. funding for the war — on Friday alone, the Netherlands, Finland and Lithuania all announced new defense assistance. The largest amount came from the Dutch government, which pledged to send more than $2.1 billion next year.

The Belgian government has also announced that it would give Ukraine nearly $1.85 billion next year from taxing the proceeds from frozen Russian assets that are currently being held by financial institutions headquartered in Belgium.

And President Volodymyr Zelensky of Ukraine praised Berlin’s plans to double its military support to the war, saying on Wednesday that “the relationship between Ukraine and Germany will become one of the most reliable pillars of all of Europe.”

Germany is now the second-largest supplier of military aid to Ukraine, according to data released by the Kiel Institute from July, the most recent available. (On Friday, Germany’s government temporarily paused discussions over its 2024 budget to deal with an unrelated court ruling, but experts said the aid to Ukraine was not expected to be affected.)

Europe is also newly poised to supply Ukraine with one of the weapons it needs most: 155-millimeter caliber shells that are fired from the howitzers and that are the backbone of Ukraine’s military.

Despite the assumed failure of the campaign by E.U. member states and Norway to donate one million of the rounds, officials and experts said just making the promise to provide the ammunition has helped revitalize Europe’s defense industry.

Building capacity to produce ammunition in Europe has improved so significantly that “there might be parity” with American output by the end of next year if projections hold steady, said Camille Grand, who was NATO’s assistant secretary general for defense investment early in the war.

How that might happen depends on somewhat murky production estimates that European executives and American officials have released.

In Europe, where there is no overarching defense coordinator, weapons manufacturers are generally reluctant to reveal their annual production numbers. A major exception is the German firm Rheinmetall, one of the West’s largest ammunition manufacturers. It predicts it will be able to produce at least 600,000 155-millimeter rounds annually by the end of 2024, up from 450,000 earlier this year.

BAE Systems, the giant British military contractor, aims to increase production of 155-millimeter shells by eight times its prewar levels by 2025, although the company will not provide an estimate of how many rounds that could be. Other European ammunition manufacturers, including Norway-based Nammo and Nexter in France, are boosting their output by tens of thousands of shells.

Taken together, Mr. Grand said, Europe could produce in the high hundreds of thousands of 155-millimeter ammunition rounds by the end of 2024 — up from about 230,000 rounds annually before the war began.

New U.S. Army projections show that American manufacturers aim to produce 720,000 rounds of the shells annually by the end of 2024.

Further production increases largely depend on whether Congress approves $3.1 billion that is included in the Biden administration’s overall $105 billion emergency aid proposal, said Douglas R. Bush, an assistant secretary of the U.S. Army and the service’s acquisition chief.

In a Nov. 7 briefing in Washington, Mr. Bush said the additional money would boost American production of 155-millimeter ammunition to as many as 80,000 rounds each month in the first half of 2025, or 960,000 annually.

Only some of the ammunition that is ultimately produced, both in the United States and Europe, would be sent to Ukraine as allies rebuild their own stockpiles. But increasing production is a necessary first step to supplying Ukraine and bolstering European security.

Mr. Grand, now at the European Council on Foreign Relations, said the possibility that an aid-cutting Republican presidential candidate would defeat Mr. Biden was a main driver of the continuing European scramble — particularly as some recent polls have shown former President Donald J. Trump drawing strong support in a theoretical rematch with Mr. Biden. As president, Mr. Trump had a dim view of NATO and had planned to withdraw thousands of American troops from Europe before Mr. Biden was elected in 2020 and halted the move.

“We need to be in that mind-set of capability,” Mr. Grand said. “And those decisions need to be taken now — not when Trump is re-elected.”

Christopher F. Schuetze contributed reporting from Berlin, Aurelien Breeden from Paris and Claire Moses from London.