The breach -- which has sparked a criminal investigation by the Department of Justice -- includes classified information about Ukraine's battle against invading Russian forces, as well as secret assessments of US allies.

Two members of a group on online social platform Discord told The Post that hundreds of pages of material had been posted there by a man who told them he worked on a US military base and brought the documents home with him.

The man, who went by the nickname "OG", regularly posted documents on the group for months, the group members told The Post, speaking on condition of anonymity.

Some information in the documents was so sensitive it was marked "NOFORN", meaning it was not to be shared with foreign nationals, The Post reported.

The group of around 24, including people from Russia and Ukraine, bonded over their "mutual love of guns, military gear and God", and formed an "invitation-only clubhouse in 2020 on Discord", The Post reported.

OG told the group members that he spent "some of his day inside a secure facility that prohibited cellphones and other electronic devices," the paper reported.

He also told them that he "toiled for hours writing up the classified documents to share with his companions in the Discord server," the paper reported.

- Security risk -

Dozens of photographs of documents have been found on Twitter, Telegram, Discord and other sites in recent days, though some may have circulated online for weeks, if not months, before they began to receive media attention.

US officials have not publicly confirmed that the materials shown in photographs posted on social media and other sites are genuine, and their authenticity could not immediately be independently verified.

Nevertheless, the leak has created headaches for Washington, exposing US concerns over the viability of a coming Ukrainian counteroffensive against Russian forces and suggesting it conducted spying on staunch allies Israel and South Korea.

Many of the documents are no longer available on the sites where they first appeared, and the United States is reportedly working to have them removed.

The fallout from the apparent leak could be significant -- even deadly -- potentially putting US intelligence sources at risk, while giving the country's foes valuable information.