FBI agents on Thursday arrested a 21-year-old national guardsman suspected of being behind a major leak of sensitive US government secrets -- including about the Ukraine war.

Broadcast live on TV networks, the dramatic arrest was the culmination of a week-long investigation -- fuelled by frenzied media coverage -- into one of the most damaging leaks of classified information since the 2013 dump of National Security Agency documents by Edward Snowden.

Addressing a news briefing, US Attorney General Merrick Garland named the suspect as Jack Teixeira, an employee of the US Air Force National Guard and the reported leader of the online chat group where the document trove first emerged.

Arrested "in connection with an investigation into alleged unauthorized removal, retention and transmission of classified national defense information," Teixeira was taken into custody without incident by FBI agents, Garland said.

News footage of the operation, in the North Dighton area of Massachusetts, showed the suspect -- dressed in red shorts and a t-shirt with his hands behind his head -- backing slowing toward rifle-armed, camouflage-clad law enforcement personnel who took him into custody.

Police in the small town in the northeastern state sought to reassure the community about the heavy law enforcement presence, saying in a statement that there was "no threat to public safety."

Two posts on the Facebook page of the Massachusetts Air National Guard's 102nd Intelligence Wing mentioned Teixeira, including one from July 2022 that said he had been promoted to airman first class -- the third-lowest enlisted rank for air force personnel.

Teixeira was expected to make an initial appearance as early as Friday at the US District Court for the District of Massachusetts.

The Pentagon has said the leak presents a "very serious" risk to US national security, and spokesman Brigadier General Pat Ryder on Thursday condemned it as a "deliberate criminal act."

In response, the Defense Department is "examining and updating distribution lists, assessing how and where intelligence products are shared, and a variety of other steps," Ryder told journalists.

Teixeira's arrest came a day 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.

Top secret material

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

The embarrassing security breach -- which included top secret documents -- 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 including Israel and South Korea.

President Joe Biden addressed the leak 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.

The alleged leaker reportedly went by the nickname "OG" and 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.

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."