The FBI has arrested a man for 'criminal' US intelligence leaks. Here's what we know so far

US Air Force National Guardsman Jack Teixeira has been arrested in Massachusetts on suspicion of leaking classified documents.

Attorney General Merrick Garland (right) said that a Massachusetts Air National Guard member was taken into custody on Thursday by federal agents. Source: AAP / Evan Vucci/AP

Key Points

• Leaked details include information on Ukraine and data on US allies.
• It is possible the leak may have started on a site called Discord.
• The Discord site hosts real-time voice, video and text chats for groups.

The FBI has arrested an employee of the US Air Force National Guard over the leaks online of classified US documents that embarrassed the country's officials with allies around the world.

US Attorney General Merrick Garland said the FBI arrested the man, Jack Teixeira, "in connection with an investigation into alleged unauthorised removal, retention and transmission of classified national defence information".

The FBI said its agents had made an arrest and were conducting "authorised law enforcement activity at a residence in North Dighton, Massachusetts".

Video images played on news channels showed heavily armed officers in Massachusetts accompanying a young man wearing a grey t-shirt and bright red shorts into a waiting car.

His head was bowed and his hair was close-cropped.

The leaks were a "deliberate, criminal act," the Pentagon said.

A military vehicle heads onto Joint Base Cape Cod where Massachusetts Air National Guardsman Jack Teixeira worked with the 102nd Intelligence Wing on the base. Source: AAP / USA TODAY Network/Sipa USA

Pentagon spokesman Brigadier General Pat Ryder referred questions about the investigation to the Department of Justice.

Some of the most sensitive leaked details are purportedly related to Ukraine's military capabilities and shortcomings, and information about US allies including Israel, South Korea and Turkey.

US President Joe Biden earlier on Thursday said investigators were closing in on the source of the online leaks.

Pentagon spokesman US Air Force Brig. Gen. Patrick Ryder during a media briefing at the Pentagon. Source: AP / Alex Brandon

The New York Times earlier reported that Mr Teixeira was a national guardsman who led Thug Shaker Central, an online forum used by about 20 to 30 people.

It is possible the leak may have started on a site called Discord, a social media platform popular with people playing online games.

The Discord site hosts real-time voice, video and text chats for groups and describes itself as a place "where you can belong to a school club, a gaming group, or a worldwide art community".

Members of the Dighton Police Department block a road leading to the area where Jack Teixeira lives, after his arrest by US authorities. Source: EPA / CJ Gunther

In one of those forums, originally created to talk about a range of topics, members would debate the war in Ukraine.

According to one member of the chat, an unidentified poster shared documents that the poster claimed was classified, first typing them out with the poster's own thoughts, then, as of a few months ago, uploading images of folded papers.

Discord has said it was cooperating with law enforcement.