US Attorney General Merrick Garland announced the arrest made "in connection with an investigation into alleged unauthorized removal, retention and transmission of classified national defense information."

Addressing a news briefing, Garland named the suspect as Jack Teixeira -- previously identified by US media as a 21-year-old airman, and the apparent leader of an online chat room where the document trove first emerged.

Garland confirmed that Teixeira is an employee of the United States Air Force National Guard, and said FBI agents took him into custody "without incident."

News helicopter footage showed the suspect, in red shorts with his hands behind his back, being placed by heavily-armed agents into an unmarked sports utility vehicle, in a forested area in North Dighton, in the northeastern state of Massachusetts.

Teixeira was due to make an initial appearance at the US District Court for the District of Massachusetts.

US media began honing in on Teixeira after The Washington Post reported that hundreds of pages of documents had been posted on the social media platform Discord by a man who worked on a US military base.

According to The New York Times, a "trail of digital evidence" pointed to Teixeira as the leader of the private chat group on Discord, called Thug Shaker Central, where the documents surfaced.

The embarrassing security breach has revealed US unease over the viability of a coming counteroffensive by Kyiv's forces against Russian troops as well as concerns about Ukrainian air defenses, and pointed to US spying on allies.

President Joe Biden addressed the leaks during a visit to Ireland, saying he was "concerned" but that the intelligence community and Justice Department were "getting close" to identifying the source of the leak.

Reports said the alleged leaker, who went by the nickname "OG," regularly posted documents in the chat group in question for months.

The group of around 24 people, including some 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," reported the Post -- which like the Times cited unidentified members of Thug Shaker Central.

- 'National security implications' -

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

He first wrote down the contents of classified documents to share with the group, but later began taking photos, telling other members not to share them, the newspaper reported.

OG had a "dark view of the government," and "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," the Post said, citing one of the group's members.

A Discord spokesperson told AFP that user safety is a priority, and that content violating its policies can result in people being banned, servers being shut down, and police alerted.

"In regards to the apparent breach of classified material, we are cooperating with law enforcement," the spokesperson said.

"As this remains an active investigation, we cannot provide further comment at this time."