The documents have allegedly been altered to reduce losses on the Russian side, US officials said, adding that their assessments are informal and separate from the investigation into the leak itself.
The first batch of documents, dated March 1 and stamped with "secret" and "top secret" markings, circulated on Twitter and Telegram.
Later on Friday, another batch of documents emerged that appeared to detail U.S. national security secrets involving areas such as Ukraine, the Middle East and China, the New York Times quoted Reuters as saying.
The U.S. Department of Justice said on Friday it was in contact with the Department of Defense and launched an investigation into the leak.
"We are aware of reports of social media posts and the [Defense] Department is investigating the matter," Pentagon spokeswoman Sabrina Singh said.
One document posted on social media estimated that 16,000 people had died since the invasion began on February 24, 2022, up to 17.5 thousand Russian forces. The US believes the real number is much higher, at around 200,000 killed and wounded Russians.
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 doubt about Ukraine's planned counter-offensive.
"These are just standard elements of Russian intelligence operational games. And nothing more," Mykhailo Podolyak said in a statement.
Source: Reuters, PAP