The Washington Post newspaper has reported that the person behind the leak of classified US government documents is a man in his 20s who worked at a US military base.

The newspaper reported the exclusive news on Wednesday, quoting a member of an invitation-only chat group with about 20 members on the "Discord" online platform.

The report says a member called "OG" began to post messages containing classified information last year.

OG is described as a young gun enthusiast, who reportedly said he spent at least some of his day "inside a secure facility that prohibited cellphones and other electronic devices."

The man reportedly rendered hundreds of classified files by hand initially, but later began uploading photos of documents themselves.

The report says some of the photos showed items and furniture in the room where OG spoke to members via video on the Discord channel.

The Washington Post says the images provide "the kind of clues that could prove useful for federal investigators."

The member who talked to the newspaper said OG took no sides in the Russia-Ukraine conflict. But OG reportedly regarded US law enforcement and the intelligence community as "a sinister force" that sought to suppress its citizens.

OG is said to have appeared very confused in the past few days.

OG is said to have asked other group members to delete any information that could possibly relate to him, including any copies of the classified documents he had shared.

The report says that to date, no federal law enforcement officials have contacted the group member who talked to the Washington Post about OG.

The US Defense and Justice Departments are investigating the leak of the sensitive and highly classified documents that have been circulating online.

They include information on the Russian invasion of Ukraine. Some documents suggest that Washington has been conducting surveillance activities on its allies and friends.