A trove of leaked Pentagon documents reveals how deeply Russia’s security and intelligence services have been penetrated by the US, demonstrating Washington’s ability to warn Ukraine about planned strikes and providing an assessment of the strength of Moscow’s war machine.

The documents paint a portrait of a depleted Russian military that is struggling in its war in Ukraine and of a military apparatus that is deeply compromised. They contain daily real-time warnings to American intelligence agencies on the timing of Moscow’s strikes and even its specific targets. Such intelligence has allowed the US to pass on to Ukraine crucial information on how to defend itself.

The documents lay bare the American assessment of a Ukrainian military that is also in dire straits. The documents, from late February and early March but found on social media sites in recent days, outline critical shortages of air defence munitions and discuss the gains being made by Russian troops around the eastern city of Bakhmut. The intelligence reports show that the US also appears to be spying on Ukraine’s top military and political leaders, a reflection of Washington’s struggle to get a clear view of Ukraine’s fighting strategies.

The material reinforces an idea that intelligence officials have long acknowledged: The US has a clearer understanding of Russian military operations than it does of Ukrainian planning. Intelligence collection is often difficult and sometimes wrong, but the trove of documents offers perhaps the most complete picture yet of the inner workings of the largest land war in Europe in decades.

American officials said while the documents offer hints about US methods to collect information on Russian plans, US intelligence agencies do not yet know if any of their sources of information will be cut off as a result of the leak. American officials have conceded they have lost some sources of information since the war began, but the new documents appear to show that America’s understanding of Russian planning remains extensive.

But the leak has the potential to do real damage to Ukraine’s war effort by exposing which Russian agencies the US knows the most about, giving Moscow a potential opportunity to cut off the sources of information.

The leak has already complicated relations with allied countries and raised doubts about America’s ability to keep its secrets. After reviewing the documents, a senior western intelligence official said the release of the material was painful and suggested that it could curb intelligence sharing. For various agencies to provide material to each other, the official said, requires trust and assurances that certain sensitive information will be kept secret.

The documents could also hurt diplomatic ties. The newly revealed intelligence documents also make plain that the US is not just spying on Russia, but also its allies. While that will hardly surprise officials, making such eavesdropping public always hampers relations with key partners, like South Korea, whose help is needed to supply Ukraine with weaponry.

Senior US officials said an inquiry, launched Friday by the FBI, would try to move swiftly to determine the source of the leak. The officials acknowledged that the documents appear to be legitimate intelligence and operational briefs compiled by the Pentagon’s Joint Staff, using reports from the government’s intelligence community, but that at least one had been modified from the original at some later point.

One senior US official called the leak “a massive intelligence breach”, made worse because it lays out to Russia just how deep American intelligence operatives have managed to get into the Russian military apparatus. Officials within the US government with security clearance often receive such documents through daily emails, one official said.

The documents show that nearly every Russian security service appears penetrated by the US in some way. For example, one entry, marked top secret, discusses the General Staff’s plans to counter the tanks Nato countries were providing to Ukraine, including creating different “fire zones” and beginning training of Russian soldiers on the vulnerabilities of allied tanks.