The largest U.S. military leak in a decade is a serious blow to Ukraine’s war effort, creating an intelligence threat for that country’s forces ahead of an expected counteroffensive this spring. 

Classified documents leaked on social media offer extensive details about munitions, training and air defense systems at a critical point in the fight. The Pentagon is still reviewing the validity of the documents.

Dozens of classified U.S. and NATO documents, some labeled “Top Secret,” began leaking in more obscure parts of the internet in January before spilling over to Twitter and Telegram and picking up attention last week.

While they only provide a status of the conflict up to March, the material does reveal insight into Ukraine’s military capabilities, including battalion sizes, training on advanced weaponry and deployment of heavy combat vehicles, such as Leopard II tanks.

They also give a view into Kyiv’s shortcomings, with at least one document describing how the country could soon run out of munitions for Soviet-era anti-air missile systems, exposing a potential vulnerability in Ukraine’s air defense systems.

Kurt Volker, a distinguished fellow with the Center for European Policy Analysis, said the leak is worrying because it gives the world a “snapshot” of U.S. assessments and judgements on the war in Ukraine.

“It is signaling to Ukrainians, to Russians, to others, ‘Here’s what we’re thinking,’” Volker said, and it “may give some clues as to the quality of our information, where we’re getting it from … which will cause the people we’re collecting on to shut that down.”

Perhaps the most alarming leak contains information on the Ukrainian air defenses.

One document, dated in February, says missiles for the S300 will run out by May, while the SA-11 Gadfly missile system will be depleted by the end of March. Both systems make up 89 percent of Ukraine’s air defenses, according to NATO, and are crucial in fending off frequent Russian missile strikes.

Russian military bloggers have already widely spread the leaked documents, including those estimating how many air defense systems and aircraft such as fighter jets are deployed by Ukraine.

NATO’s assessment also says Ukraine could withstand only a few more waves of Russian missile strikes, while providing a map of where its air defense systems are.

John Herbst, senior director of the Atlantic Council’s Eurasia Center and a former U.S. ambassador to Ukraine, referred to the information on Ukraine’s air defenses as the “most unfortunate” part of the leaked documents.

But he said none of the documents contain any critical information not already known by NATO allies and Russian intelligence.

“I don’t think there’s any doubt that some damage has been done to Ukraine’s war effort as a result of leaks,” he said. “But was the damage overwhelming? Probably not.”

Other documents explain the strength and capabilities of Ukrainian brigades, as well as which weapons systems Ukraine has trained on. One February document estimates the number of tanks, infantry fighting vehicles and artillery units fielded by Ukraine.

Moscow has been unusually quiet on the leaks, though state-run news outlet TASS published the details of the documents. 

Russian military bloggers appear to be skeptical, with one account, the War Chronicle, pointing out misspellings and errors in the material.

Rybar, a blogger with more than 1 million followers on Telegram, called it “a controlled leak and a massive disinformation campaign” to make Ukrainians look unprepared and ultimately encourage Russia to make a mistake.

The leak is also worrying for the U.S. because Russia could figure out how the Pentagon is collecting crucial intelligence on Russian forces and capabilities.

The documents contain information on not only Ukraine’s forces but also a detailed assessment on Russia’s army, from tanks to artillery and aircraft.

Michael O’Hanlon, a senior fellow at the Brookings Institution, said in an email that the leaks could “lead Russia to tighten its communications security, reducing our knowledge of their next moves.”

It’s unclear how many documents are currently circulating across the internet, but at least 100 separate documents have appeared online, according to several reports and analysts.

The Pentagon on Monday declined to comment on the size and scope of the leak, revealing that the Defense Department was still investigating the matter.

“The Department of Defense is working around the clock to look at the scope and scale of the distribution, the assessed impact and our mitigation measures,” Chris Meagher, assistant to the secretary of Defense for public affairs, told reporters.  

“We’re still investigating how this happened, as well as the scope of the issue. There have been steps to take a closer look at how this type of information is distributed and to whom. We’re also still trying to assess what might be out there,” he added. 

Meagher also revealed that the format of the documents are “similar in format to those used to provide daily updates to our senior leaders on Ukraine and Russia-related operations, as well as other intelligence updates,” and “present a very serious risk to national security.”

Investigative group Bellingcat traced the documents back to a leak in early March through a channel on Discord, a website where users discuss topics, primarily gaming. 

Ten of the documents appeared on a Discord channel related to the popular video game “Minecraft” in the earliest verified leak, but Bellingcat said there may have been leaks as early as January on a separate Discord server.

The Pentagon last week said it was reviewing the document leaks and had turned over a formal criminal investigation of them to the Department of Justice. U.S. officials on Monday said the investigation was a “top priority” and urged caution when studying the documents, noting some of them have been altered. Ukrainian prime minister arrives in DC at make-or-break moment ‘Pissed off as hell’: US tries to contain leaks fallout with friends and enemies

The Defense Department did not identify how many have been altered. But a document that gained attention last week inflated the number of Ukrainians killed in action and drastically reduced the number of Russian casualties in the war.

Mykhailo Podolyak, an adviser to the head of Ukraine’s presidential office, has called the intelligence leaks an attempt to “divert attention” and “sow discord” among Western allies.

Also on Monday, the Pentagon would not explain whether more documents are likely to appear online and how many officials in the Defense Department had access to these documents. Officials say the leak has prompted the department to take steps to review how some classified information is shared and to whom.