Classified war documents detailing secret U.S. and NATO plans for building up the Ukrainian military before a planned offensive against Russia were posted this week on social media channels, senior U.S. officials said.

The Pentagon is investigating who may have been behind the leak of the documents, which appeared on Twitter and on Telegram, a platform with more than half a billion users that is widely available in Russia.

Military analysts said the documents appear to have been modified in certain parts from their original format, overstating U.S. estimates of Ukrainian war dead and understating estimates of Russian troops killed.