Secret documents that provide details of US and NATO plans to help prepare Ukraine for a spring offensive against Russia have spilled onto social media platforms, the New York Times reported on Thursday.

The Pentagon said it is assessing the reported 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.

Information in the documents is at least five weeks old, with the most recent dated March 1, the report said.

One of the documents summarized the training schedules of 12 Ukraine combat brigades, and said nine of them were being trained by US and NATO forces, and needed 250 tanks and more than 350 mechanized vehicles, the newspaper said.

The 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 rocket systems, the US-made artillery rocket systems that have proven highly effective against Russian forces, it added.

The report quoted military analysts who warned that some documents appear to have been altered in a disinformation campaign by Russia, in one document inflating Ukrainian troop deaths and minimizing Russian battlefield losses.