Some of the military documents published on social media, which US officials say are authentic, expose the extent of US eavesdropping on key allies, including South Korea, Israel and Ukraine, CNN has reported.
Others divulge key weaknesses in Ukrainian weaponry, air defence, and battalion sizes and readiness at a critical point in the war, as Ukrainian forces gear up to launch a counteroffensive against the Russians, it added.
Ukraine has already altered some of its military plans because of the leak, a source close to Ukrainian President Volodymyr Zelenskyy told CNN.
The Pentagon has stood up an "interagency effort" to assess the impact of the leak, Pentagon deputy press secretary Sabrina Singh said Sunday.
"The Department of Defense continues to review and assess the validity of the photographed documents that are circulating on social media sites and that appear to contain sensitive and highly classified material," she noted.
The documents appeared online last month on the social media platform Discord, according to screenshots of the posts reviewed by CNN. The posts are photos of crumpled documents laid on top of magazines and surrounded by other random objects, such as zip-close bags and Gorilla Glue. It is as if they had been hastily folded up and shoved into a pocket before being removed from a secure location, a source familiar with these kinds of documents told CNN.
A Discord spokesperson confirmed in a statement Sunday that they are cooperating with law enforcement on the investigation.
CNN has reviewed 53 leaked documents, all of which appear to have been produced between mid-February and early March.
One document reveals that the US has been spying on Zelenskyy. That is unsurprising, said the source close to Zelenskyy, but Ukrainian officials are deeply frustrated about the leak.