Boston, Massachusetts - Jack Teixeira, the US airman accused of leaking top-secret Pentagon documents in an online chat room, was slapped with additional federal charges on Thursday.

The 21-year-old Air National Guard IT specialist allegedly orchestrated the most damaging leak of US classified documents in a decade.

A federal grand jury in Boston indicted him on six counts of "willful retention and transmission of classified information relating to the national defense."

They each carry sentences of up to 10 years in prison.

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The charges are in addition to the two counts filed by prosecutors against Teixeira after his arrest in April.

Teixeira is suspected of posting the documents – some dated as recently as early March – to a private chat group on Discord.

Some of the files later appeared on other sites, including Twitter, 4Chan, and Telegram.

The documents, which soon spread across the internet, pointed to US concern over Ukraine's military capacity to fight off the invading Russian forces and showed Washington had apparently spied on allies Israel and South Korea, among other sensitive details.

It was the biggest such breach since the 2013 dump of National Security Agency documents by Edward Snowden and raised tough questions about access by Teixeira, a junior staffer, to high-level secrets.

Teixeira was an airman first class, the third-lowest rank for enlisted US Air Force personnel and had possessed a top secret security clearance since 2021.

Last month a judge ordered that Teixeira remain in jail pending trial after prosecutors argued that he posed an ongoing risk to American national security.

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Prosecutors had argued that Teixeira might still have access to classified documents and that "hostile" nations could aid his escape if he was released from prison, also saying he had a history of making "violent" statements.

Teixeira wrote on social media in November that he wanted to "kill a ton of people" because it would be "culling the weak minded," the prosecution wrote in a court document.

Prosecutors also said the airman sought advice from another user about what type of rifle would be easy to operate from the back of an SUV, and that he searched mass shootings online.

Teixeira's defense team said their client no longer had access to classified material and that the government was exaggerating the threat he posed, arguing he should be released into his father's custody pending trial.