Russia or pro-Russian elements are likely behind the leaking of several classified US military documents published on social media that provide information about the war in Ukraine, three US officials told Reuters.

The documents appear to have been altered to downplay the actual losses of Russian forces, US officials said, adding that their estimates were informal and separate from the investigation into the leak itself.

Officials spoke to Reuters on condition of anonymity, given the sensitivity of the matter and declined to discuss the documents in any detail. The Pentagon, for its part, declined to comment on the authenticity of the documents, which are dated March 1 and carry markings indicating they are classified as “Secret” and “Top Secret.”

A document posted on social media said 16,000 to 17,500 Russian forces had been killed in the invasion. The United States believes the real number is much higher. Some 200,000 Russians are believed to have been killed and wounded, officials say.