A leak of classified documents detailing secret U.S. and NATO plans to help Kyiv looks like a Russian disinformation operation to sow doubts about Ukraine's planned counter-offensive, a Ukrainian presidential official said on Friday.

Mykhailo Podolyak told Reuters that the leaked data reported by the New York Times contained a "very large amount of fictitious information" and that Russia was trying to seize back the initiative in its invasion.

"These are just standard elements of operational games by Russian intelligence. And nothing more," he said in a written statement.

Reuters was not immediately able to review the leaked documents.

The New York Times reported on Thursday that the Pentagon was investigating how documents detailing plans to build up the Ukrainian military for a planned offensive had been posted on social media channels this week.

"Russia is looking for any ways to seize back the initiative," Podolyak said. "To try to influence the scenarios for Ukraine's counteroffensive plans. To introduce doubts, to compromise the ... ideas, and finally to intimidate (us) with how 'informed' they are."

Ukrainian troops have faced for months a Russian onslaught in its east concentrated around the city of Bakhmut. It hopes to launch a counter-offensive in the coming weeks or months to wrest back territory held by Russia.

The Times report said the documents appeared to have been modified in certain parts. One part offered an estimate for Ukrainian military losses that was far higher than Western estimates made available so far.

Ukraine does not disclose the scale of its losses and is very sensitive about the subject. (Reporting by Pavel Polityuk; writing by Tom Balmforth; editing by Jason Neely and Mark Heinrich)