Jack Teixeira, the US airman accused of leaking top-secret Pentagon documents in an online chat forum, pleaded not guilty on Wednesday to charges that could send him to prison for decades.

The 21-year-old Massachusetts Air National Guard IT specialist was arrested in April for allegedly orchestrating the most damaging leak of US classified documents in a decade.

He is facing six counts of retaining and transmitting national defense information. They each carry sentences of up to 10 years in prison.

Teixeira appeared before Magistrate Judge David Hennessy in Worcester, Massachusetts, on Wednesday and entered a plea of not guilty to each of the charges, according to court documents.

His lawyer asked that Teixeira be released pending trial, but the judge denied the request.

Teixeira is suspected of posting the documents -- some dated as recently as early March -- 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.

The documents, which soon spread across the internet, pointed to US concern over Ukraine's military capacity against Russian forces and showed Washington had reportedly 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.

Internal Air Force documents indicated that his superiors regularly cautioned him about unlawfully accessing confidential material.

Teixeira's lawyers have pointed out in court that their client did not anticipate secret material he put on Discord to be widely circulated on the internet, chastising prosecutors for exaggerating the threat Teixeira represents to national security.

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

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

Teixeira was detained on April 13 in a dramatic arrest broadcast live on TV networks.

Read more: Pentagon leaks began hours after start of Ukraine war: NYT