A leaked trove of classified documents outlining American and NATO war planning in Ukraine has sparked a US military investigation, the New York Times (NYT) has reported.

The briefing presentation slides, some marked “top secret,” show assessed troop strengths, timelines for arms shipments and ammunition expenditures, among other data.

“We are aware of the reports of social media posts and the department is reviewing the matter,” Pentagon spokesperson Sabrina Singh told the outlet.

Officials said they are working to have the material removed from the internet, but had not succeeded as of Thursday night.

One of the documents marked “top secret” provides the “Status of the Conflict as of 1 Mar.,” suggesting the slides are several weeks old.

Though they do not describe any specific battle plans, the files offer a glimpse into US intelligence assessments for Russian and Ukrainian troop levels, timetables for training and equipment deliveries, weather data, as well as ammunition expenditures – including for the US-supplied HIMARS platform.

The Pentagon has not previously disclosed the rate at which Ukrainian forces are using munitions for the weapon.

Another file lists 12 new Ukrainian combat brigades – which are usually made up of 4,000 to 5,000 troops – noting that the US and NATO bloc are training and supplying nine of them. Of those, the document states that six would be ready by the end of March, and the remaining three by April 30.

If genuine the documents represent one of the most significant intelligence breaches since the beginning of the Russia-Ukraine conflict, and come as Kyiv is reportedly preparing a major counter-offensive against Russian forces.

One of the documents shows two possible routes into the Donbass, Zaporozhye and Kherson regions.

It is unclear when the counter-attacks are set to begin, but Ukrainian Maj. Gen. Kyrylo Budanov recently claimed that the two sides would fight a “final” and “decisive battle” sometime in the spring.