US officials have not confirmed the authenticity of the documents, images of which have circulated on social media platforms for weeks and which have sparked an uproar amongst some US allies since receiving media attention in recent days.

Two members of a group on the 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 newspaper, 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 newspaper reported.

- 'Dark view of government' -

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.

Later, he began taking photos of the documents and shared those with the group.

"When rendering hundreds of classified files by hand proved too tiresome, he began posting hundreds of photos of documents themselves," The Post reported.

OG had told the other members of the group not to share the documents, and that he had not intended to be a whistleblower, The Post reported, citing one of the friends.

The Post said OG had "a dark view of the government."

"(A group member) said he spoke of the United States, and particularly law enforcement and the intelligence community, as a sinister force that sought (to) suppress its citizens and keep them in the dark. He ranted about 'government overreach.'"

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.

The breach has sparked a criminal investigation by the US justice department.

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

The Pentagon says the breach posed a "very serious risk to national security."

The leaks have exposed US concerns over the viability of a coming Ukrainian counteroffensive against Russian forces, and suggested it spied on allies Israel and South Korea.

White House national security spokesman John Kirby said that Washington was contacting allies and partners at "very high levels" in the wake of the release of the documents.