Pentagon officials are reportedly racing to identify the source of a leak of military intelligence, which could disrupt Ukraine’s planned spring offensive against Russian troops.

Scores of slides were published on Discord and Telegram, with some dating back to January, according to The Economist.

The Pentagon is reported to be leading a cross-government panel to assess what damage has been done, while the Department of Justice has opened a probe into the source of the leak. Senior officials are apparently trying to reassure their counterparts in allied countries.

The documents are said to contain detailed intelligence that could be of significant strategic use to the Russian military, such as the precise quantity of armor and artillery held by each Ukrainian brigade, the state of its air defenses and location of missile batteries.

Read more on US military leaks: US Fears a Second Snowden May be Leaking Secrets.

The slides themselves, which have been reviewed by defense experts and assessed to be legitimate, are photographs of paper documents.

Some reportedly appear as if assembled into a briefing packet by the Joint Staff’s intelligence arm (J2), while others contain marks describing how the intelligence was obtained. These include “SI-G” – indicating information gleaned from signals intelligence like wire taps.

Although some of the documents were altered before uploading to Telegram, to inflate Ukrainian casualties and understate Russian ones, most have reportedly not been manipulated.

While the leaked information also reveals the parlous state of Russia’s military, the leaks could sow discord among the US and its allies at a critical moment of the war in Ukraine. There are reported details of how many special forces operatives from allied countries like France and the UK are currently training Ukrainian troops, for example.

The leaks call to mind the files released by notorious US whistleblower and former NSA contractor Edward Snowden, who is currently living in Russia. However, while his files were passed on to media outlets to parse and report on, the latest leak went straight up on social media.