By Phil Stewart
WASHINGTON (Reuters) -Russia or pro-Russian elements are likely behind the leak of several classified U.S. military documents posted on social media that offer a partial, month-old snapshot of the war in Ukraine, three U.S. officials told Reuters on Friday, while the Justice Department said separately it was probing the leak.
The documents appear to have been altered to lower the number of casualties suffered by Russian forces, the U.S. officials said, adding their assessments were informal and separate from the investigation into the leak itself.
The U.S. officials spoke on condition of anonymity given the sensitivity of the matter and declined to discuss the documents in any detail.
The Kremlin and Russia's embassy in Washington did not immediately respond to requests for comment.
An initial batch of documents circulated on sites including Twitter and Telegram, dated March 1 and bearing markings showing them classified as "Secret" and "Top Secret."
Later on Friday, an additional batch appearing to detail U.S. national security secrets pertaining to areas including Ukraine, the Middle East and China surfaced on social media, the New York Times reported.
Reuters was not able to verify the authenticity of the documents.
The U.S. Justice Department said late on Friday it was in touch with the Defense Department and began a probe into the leak. It declined further comment.
A leak of such sensitive documents is highly unusual.
"We are aware of the reports of social media posts and the Department (of Defense) is reviewing the matter," Pentagon spokesperson Sabrina Singh said.
A CIA spokesperson said the agency was also aware of the posts and was looking into the claims.
One document posted on social media said 16,000 to 17,500 Russian forces had been killed since Russia's Feb. 24, 2022, invasion of Ukraine.
The United States believes the actual figure is much higher, at around 200,000 Russians killed and wounded, officials have said.
Ukrainian forces repelled Russia's initial advance on Kyiv early in 2022, and the conflict, which Moscow calls a "special military operation," has become one of grinding trench warfare in the east and south.
Speculation has been mounting about what offensives Kyiv and Moscow might attempt in the war's second year. The leaked documents did not appear to offer any specific insight into Kyiv's war plans.
A Ukrainian presidential official said on Friday that the leak contained a "very large amount of fictitious information" and looked like a Russian disinformation operation to sow doubts about Ukraine's planned counter-offensive.
"These are just standard elements of operational games by Russian intelligence. And nothing more," Mykhailo Podolyak said in a written statement.