By Helene Cooper, Michael Schwirtz and Thomas Gibbons Neff

WASHINGTON — For more than a year, Ukrainian air defences, reinforced by Western weaponry, have kept Russian planes at bay.

But without a huge influx of munitions, Ukraine’s entire air defence network, weakened by repeated barrages from Russian drones and missiles, could fracture, according to US officials and newly leaked Pentagon documents, potentially allowing Russian President Vladimir Putin to unleash his lethal fighter jets in ways that could change the course of the war.

In the early days of the invasion, Russian aircraft flew hundreds of combat flights to bomb targets in Ukraine. But a combination of quick thinking by Ukrainian commanders and poor intelligence and bad aim by Russian pilots left many of Ukraine’s warplanes and air defences intact, preventing Moscow from gaining control of the skies above the battlefield and forcing Russia to keep much of its air force out of the fight.

Now, Pentagon officials are worried that Moscow’s barrage of attacks from afar is draining Ukraine’s stores of the missiles it uses to defend itself. And a Pentagon assessment from late February contained in the trove of leaked documents that were discovered circulating online last week paints an even grimmer picture.

Stocks of missiles for Soviet-era S-300 and Buk air defence systems, which make up 89% of Ukraine’s protection against most fighter aircraft and some bombers, were projected to be fully depleted by May 3 and mid-April, according to one of the leaked documents. The document, which was issued Feb. 28, based the assessment on consumption rates at the time. It is not clear if those rates have changed.

The same document assessed that Ukrainian air defences designed to protect troops on the front line, where much of Russia’s air power is concentrated, will “be completely reduced” by May 23, resulting in strains on the air defence network deeper into Ukrainian territory.

If that happens, officials say, Moscow could decide it is finally safe for its prized fighter jets and bombers to enter the fray and directly threaten the outcome of the war on the ground.

Senior Pentagon officials say that such a move would be a major challenge for Ukraine, particularly if Russian fighter jets and bombers are given freer rein to attack Ukrainian troop positions and essential artillery targets on the ground.

In a move to shore up Ukraine’s air defences, the Biden administration announced last week that it would send additional air defence interceptors and munitions as part of a $2.6 billion aid package, part of which will be used to help Kyiv prepare for a planned spring offensive against Russian troops. Whether that will be enough depends, officials say, on a number of factors, including whether NATO allies make their own deliveries, and whether Putin continues to decline to risk his valued warplanes.

The downing of an US drone by a Russian fighter jet over the Black Sea last month exacerbated fears that the Kremlin is looking for ways to use its air force in the war. Russia still has considerable air capability, with about 900 fighter jets and about 120 bombers, according to the World Directory of Modern Military Aircraft.

“The Russian army has been mauled,” Gen. Mark Milley, chair of the US Joint Chiefs of Staff, said in an interview with MSNBC’s ‘Morning Joe’ in February. “But the Russian air force has not.”

Indeed, an assessment in another leaked Pentagon document puts the number of Russian fighter jets currently deployed in the Ukraine theatre at 485 compared with 85 Ukrainian jets.

Many experts expected the Russian air force, with its backbone of MIG and newer generation Sukhoi jets, to be a deciding factor in the first months of the war. But it has been marginalized against a much smaller Ukrainian force because of Ukraine’s intact air defences and Russian tactical and strategic blunders.

Ukraine reorganized its mobile surface-to-air missile batteries after the first three days of the war and shot down several Russian Su-34s and other attack planes last year. The Ukrainian batteries fired their missiles and then quickly scooted away to different locations so that Russia would not detect their positions and return fire.

With his prized warplanes getting shot down, Putin pulled them back. For much of the war, these jets and ground attack aircraft such as the Su-25 have concentrated on sorties along the front lines, lobbing rockets at Ukrainian positions, as well as long-range missile attacks carried out from Russian or Belarusian territory.

“They made the choice that they were not going to sacrifice their knights for their pawns,” said Dara Massicot, a senior policy researcher at the Rand Corp. “Instead, they’re going to throw those mobilized troops without proper air support, because they’re a more plentiful resource.”

Since those early days, Ukrainian air defences have held off an onslaught of Russian missile and drone attacks. But those systems, according to US military officials and the leaked documents, are rapidly depleting, potentially offering a window for Russian planes to do severe damage.

Air defences are layered, with different types of weapons designed to intercept aircraft and missiles flying at different altitudes — from low-flying helicopters to high-altitude bombers and cruise missiles. In Ukraine, these defensive weapons have also been used to target drones and cruise missiles as Ukrainian forces have tried to defend their cities from Russia’s campaign against the country’s infrastructure.

They are like a Jenga tower: Once you take out one piece, the rest are vulnerable. If Ukraine’s air defences collapse or are significantly reduced, their ground forces, in particular their artillery, will be under immediate threat. And if Ukraine is without artillery — the backbone of the war effort — Russian forces will have an opportunity to make significant gains on the battlefield.

Yurii Ihnat, a spokesperson for Ukraine’s Air Force Command, did not deny Ukraine was suffering from depleted stocks of air defence munitions, but he said new systems delivered by Western partners could fully replace what had been used up.

“The question is numbers,” he said in a text message. “To fully replace them, we need many systems, and I won’t tell you how many.”

A US defence official said that the Pentagon was alarmed by Ukraine’s current air defence degradation and that it had been a persistent concern for months.

Another senior US military official said that reinforcing and replacing these systems would be crucial to helping Ukraine regain territory in the spring.

“It’s a pretty high-risk missile mission, you know, to fly into the heart of an air defence and then try to defeat it,” said Gen. Philip Breedlove, a former US fighter pilot who was the supreme allied commander for Europe. “So Russia still remains a bit fearful of flying into Ukraine because there’s still a fair amount of density of Ukrainian kit, and slowly, ever so slowly, way too slowly, the West is beginning to send even better kit and more kit.”

So, as the United States and European countries rush tanks, fighting vehicles and ammunition to Ukraine, they have also stepped up efforts to reinforce the country’s air defences. They have provided not only missiles for Ukraine’s existing systems, such as its Soviet-era S-300s, but new and updated systems.

Pentagon officials say a key part of their quest to help Ukraine now is to make sure that it can continue to keep Russian pilots out of the fight. A senior military official said the administration and the West must convince Putin, by their upgrading Ukraine’s air defences, that if he decides to go for broke, he will lose a pillar of his military.

Even without using his air force, Putin has launched so many missiles that Ukraine has depleted its air defences shooting them down. US officials worry that Moscow might now decide the battlescape is safe enough to send its fighter jets and bombers to join the fight.

“The weekly salvos of cruise missiles that Russia has been launching have, in many instances, been met with salvos of defensive interceptors, and that has an effect of soaking up capacity,” said Tom Karako, a senior fellow at the Centre for Strategic and International Studies and the director of its Missile Defence Project. “You don’t have to be a math major to start doing some arithmetic and know that the NASAMS and Hawks and other scarce air defence interceptors are continually under strain.”

For the first months of the war, Ukraine relied heavily on the S-300 and the Buk — both mid- to long-range surface-to-air systems — to target aircraft, cruise missiles and ballistic missiles.

Western countries have started providing Kyiv with more sophisticated systems. In October, Germany began sending IRIS-T air defence batteries, which each consist of a radar, a command-and-control system and three missile launchers, carrying a total of 24 missiles.

In November, Ukraine received its first shipment of NASAMS — for National Advanced Surface-to-Air Missile System — which is jointly produced by the United States and Norway. Each NASAMS includes a radar, sensors, launchers that can be loaded with six missiles each and a mobile command center where soldiers can monitor airborne threats.

And this month, several dozen Ukrainian soldiers are wrapping up their training on the Patriot missile system. The Ukrainian troops will be deployed to the front lines, armed with the most advanced US ground-based air defence system. The Patriot is mobile, in theory. But it comes with a pretty large footprint, and Russia has already promised that it will target it.

But Ukraine will need more — far more — than the Patriot in the coming months, military officials say, and Pentagon procurement officials have been scouring allied stockpiles.

Several US officials said that despite fears that the Russian air force could pounce, such a move could be risky for Putin.

-New York Times