A 21-year-old US airman will appear in court on Friday in connection with a leak of highly classified military documents about the Ukraine war and other national security issues.

Jack Teixeira was arrested by the FBI at his family home in Dighton, rural Massachusetts, on Thursday.

Dressed in shorts and a T-shirt, he was led away by heavily armed agents.

Mr Teixeira has been identified as the leader of an online chat group where the documents first emerged.

He is charged with the unauthorised removal and transmission of classified information. He will appear in court in Boston, Massachusetts later on Friday.

Dozens of leaked documents revealed US assessments of the war in Ukraine as well as sensitive secrets about American allies, embarrassing Washington and raising fresh questions over the security of classified information.

Mr Teixeira worked as an IT specialist in the intelligence wing of the Massachusetts National Guard, based at Otis Air National Guard Base in western Cape Cod.

The National Guard is a reservist wing of the US Air Force. They are not employed full time in the military, but can be deployed when necessary.

Mr Texeira's official title is Cyber Transport Systems journeyman and he holds the rank of Airman 1st Class - a relatively junior position.

It is not clear what level of security clearance Mr Texeira had, but according to the Air Force website, employment in the role requires a single scope background investigation (SSBI). That clearance is reportedly required for access to top secret information.

It was not until intelligence material was posted outside the chat room group that Pentagon officials became aware of the leak, prompting a massive search for the culprit.

Aside from the age of the suspect, the motive is also thought to be unusual.

While Mr Teixeira is said to have harboured a scepticism of government, friends said he was neither a whistleblower nor a foreign agent.

The Pentagon says it will re-examine how classified information is distributed, but a spokesman said it was "the nature" of the US military to entrust young service members with high levels of responsibility.

Footage of the arrest in Dighton, a town of 8,000 people about an hour south of Boston, shows a young man walking backwards with his hands raised to armed FBI officers. He was handcuffed and led to a vehicle.

US Attorney General Merrick Garland said the suspect was taken into custody without incident. He provided no further details on the investigation or the motive for the leaks.

At a separate news conference earlier in the day, defence department spokesman Brigadier General Pat Ryder said the leak had been a "deliberate criminal act".

Eddy Souza, a 22-year-old man who said he went to school with Mr Teixeira, told Reuters he was surprised his former classmate had been identified as the suspect in the leaks.

"He's a good kid, not a troublemaker, just a quiet guy," Mr Souza said. "It sounds like it was a stupid kid's mistake."

Mr Teixeira also oversaw an online chat room made up mainly of male teenagers, with whom he had allegedly been sharing top secret information for months.

Starting several months ago, at least 50 but perhaps more than 100 classified documents were posted on Discord - a social media platform popular with gamers.

The Post reported that the man was the leader of a Discord chatroom whose roughly two dozen members swapped "memes, offensive jokes and idle chitchat" and prayed and watched films together.

The members included people from Russia and Ukraine and a number of other countries in Europe, Asia and South America, the paper reported.

At first the leaks were kept inside the small chatroom, but in early March members began posting them on other Discord servers, including ones dedicated to the game Minecraft and a Filipino YouTuber.

From there they were posted on the fringe message board 4chan and on the Telegram chat app, particularly on pro-Russia channels. In some cases they were altered to increase Ukrainian casualty counts.

A defence department spokesman said the Pentagon was continuing to work to "understand the scope, scale and impact" of the leaks.

Republican congressman Mike Turner - the chairman of the House Intelligence Committee - vowed to "examine why this happened, why it went unnoticed for weeks, and how to prevent future leaks".