War plans spread on social media reportedly contain charts and details about weapons deliveries and other sensitive information, New York Times says.
The Pentagon said on Thursday it is assessing the apparent security breach.
“We are aware of the reports of social media posts, and the department is reviewing the matter,” Deputy Press Secretary Sabrina Singh said.
The documents were spread on Twitter and Telegram, and reportedly contain charts and details about weapons deliveries, battalion strengths and other sensitive information, the Times said.
There was no explanation as to how the plans were obtained.
Information in the documents is at least five weeks old with the most recent dated March 1, the report said. The plans did not provide specific action such as when Ukraine would launch the offensive.
One of the documents summarised the training schedules of 12 Ukraine combat brigades, and said nine were being trained by US and NATO forces. About 250 tanks and more than 350 mechanised vehicles are required for the operation, the newspaper said.
The classified documents, at least one of which carried a “top secret” label, were circulated on pro-Russian government channels, it said.
Information in the documents also details expenditure rates for munitions under Ukraine military control, including for the HIMARS rockets, the US-made artillery systems that have proven highly effective against Russian forces, it added.
“To the trained eye of a Russian war planner, field general or intelligence analyst … the documents no doubt offer many tantalizing clues and insights,” the Times said.
The report quoted military analysts who warned some documents appear to have been altered in a disinformation campaign by Russia, with one inflating Ukrainian troop deaths and minimising Russian battlefield losses.
The leak comes as fighting continues in the town of Bakhmut in eastern Ukraine, which has been one of the deadliest battles of the war so far.
While Western analysts have played down its strategic significance, Kyiv has framed its dogged defence of the city as a way of wearing down Russian forces ahead of the expected counteroffensive bolstered by advanced Western-supplied weapons.
Russian President Vladimir Putin has called the invasion a “special military operation” necessary to eliminate Ukrainian Nazis and defend Russia from a hostile West. Ukraine and its allies call it an unprovoked war of conquest.
The conflict has killed thousands of people, wrecked cities and destabilised the global economy. Millions of Ukrainians have fled to neighbouring nations while others have been internally displaced.