Secret US and Nato plans to arm Ukraine for a spring offensive have reportedly been leaked online.
The files, posted on Twitter and Telegram, appear to show maps and details about weapons deliveries.
The documents, dated between Feb 23 and March 1, also showed training schedules and locations for Ukrainian and Russian military units and artillery guns along the front line.
Ukraine claimed the papers were fake and part of a Russian disinformation campaign.
A Pentagon spokesman confirmed it was probing the security breach after it was first highlighted by the New York Times.
In a statement Friday, Sabrina Singh, a Pentagon spokeswoman, said the Defense Department "made a formal referral" of the matter to the Justice Department for investigation.
The Justice Department, in a separate statement Friday, said, “We have been in communication with the Department of Defense related to this matter and have begun an investigation.”
A Telegram channel linked closely to Russia’s FSB security services had boasted of a “first breakthrough for Russian intelligence” when it published photos of the leaked documents.
It said the five documents, marked as “top secret”, gave away valuable intelligence ahead of Ukraine’s spring counteroffensive.
It said: “In particular, there were graphs of the expected supply of weapons, the number of troops and the consumption of ammunition.”
Mykhailo Podolyak, an adviser to the Ukrainian presidential office, said the leak looked like a Russian disinformation operation, saying that it contained a “very large amount of fictitious information”.
Some commentators pointed out that the original document may have been doctored, as the latter versions posted online had a significantly greater number of Ukrainian casualties and equipment losses recorded and a massive decrease in the Russian battle damage.
A battlefield assessment dated February 23 said 43,500 Russian troops and 17,500 Ukrainians had been killed in action. Later versions showed Ukraine’s losses had increased to 71,500.
