The US airman accused of leaking a trove of top secret Pentagon documents pleaded guilty Monday under a deal that saw him sentenced to some 17 years in prison in return for prosecutors dropping more serious espionage charges.

Jack Teixeira, once a low-level Massachusetts Air National Guard IT specialist, was accused of orchestrating the most damaging leak of US classified documents in a decade, some of which concerned the war in Ukraine.

Had he faced charges under the Espionage Act, Teixeira could have been imprisoned for life.

Instead he appeared in a federal court in an orange prison jumpsuit Monday and pleaded guilty to six counts of willful transmission of national defense information.

He will be sentenced to up to 16 years and eight months under the agreement, and must pay a fine of $50,000 and assist intelligence officials to understand the extent and impact of his disclosures.

Asked by the presiding judge whether he disputed any evidence, Teixeira said he did not. Asked whether he knew the documents were classified, he responded "Yes your honor."

Teixeira was detained in April 2023 in a dramatic arrest broadcast live on TV networks.

He was accused of posting the documents -- some dated as recently as March 2023 -- to a private chat group on the social media platform Discord.

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

- 'Hostile' nations -

The documents, which soon spread across the internet, pointed to US concern over Ukraine's military capacity against invading Russian forces.

They also 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 and other junior staffers 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 year a judge ordered that Teixeira remain in jail after prosecutors argued he posed an ongoing risk to American national security.

Prosecutors said Teixeira might still have access to classified documents and that "hostile" nations could aid his escape if he was released from prison.

They also cited his history of making "violent" statements.

Teixeira wrote on social media in November 2022 that he wanted to "kill a ton of people" because it would be "culling the weak minded," the prosecution wrote in a court document.

- Online dating -

In a separate case, federal prosecutors Monday also unveiled charges against a civilian Air Force employee for transmitting classified information on an online dating platform.

David Slater, 63, was arrested in Nebraska over the weekend for sharing secret information with someone "who claimed to be a female living in Ukraine," the Department of Justice said.

The documents, some of which were classified as "top secret," concerned the war in Ukraine, including military targets and Russian military capabilities.

Slater, a former lieutenant colonel, was working as a civilian assigned to the US Strategic Command, whose portfolio includes nuclear deterrence.

He allegedly shared the information between February and April 2022.

Slater faces up to 10 years in prison if found guilty.