A US Air National Guardsman is to appear in court on Friday in Boston, Massachusetts, charged in connection with a leak of classified US military intelligence on the war in Ukraine.

Jack Teixeira, 21, was arrested by the FBI at his family home in Dighton, rural Massachusetts, on Thursday.

Dressed in shorts and T-shirt, he was led away as news helicopters hovered.

Airman 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.

The leaks revealed US assessments of the war in Ukraine, as well as sensitive secrets about American allies, embarrassing Washington and posing risks to national security.

Airman 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.

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

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 Airman Teixeira is said to have harboured a scepticism of government, friends said he was neither a whistle-blower nor a foreign agent.

He joined the National Guard in 2019, according to his service record, obtained by the BBC's US partner, CBS News.

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

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.

"There were about six to eight Army guys with rifles walking around," local resident Dick Treacy told Reuters news agency. "This is a very quiet area."

US Attorney General Merrick Garland said the suspect had been 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 Airman 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."

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.

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".