A 21-year-old member of the US Air National Guard accused of leaking top secret military intelligence records online has been charged with unlawfully copying and transmitting classified material.

Jack Douglas Teixeira of North Dighton, Massachusetts, who was arrested by heavily armed FBI agents at his home on Thursday, made his initial appearance in a crowded federal court wearing a brown khaki jumpsuit on Friday.

At the hearing, Boston's top federal national security prosecutor, Nadine Pellegrini, requested that Teixeira be detained pending trial, and a detention hearing was set for Wednesday.

During the brief proceeding, Teixeira said little, answering "yes" when asked whether he understood his right to remain silent.

The judge said Teixeira's financial affidavit showed he qualified to be represented by a federal public defender, and he appointed one.

After the hearing, three of Teixeira's family members left the courthouse, with a group of reporters trailing them for several blocks. They entered a car without making any comments.

The leaked documents at the heart of the investigation were posted online on a social media website weeks earlier, but news of their existence did not come to light until it was reported by the New York Times last week.

It is believed to be the most serious US security breach since more than 700,000 documents, videos and diplomatic cables appeared on the WikiLeaks website in 2010. The Pentagon has called the leak a "deliberate, criminal act."

US officials are still assessing the damage done by the leaks, which included records showing purported details of Ukrainian military vulnerabilities and embarrassed Washington by revealing its spying on allies.

Senate Democratic leader Chuck Schumer has requested a briefing for all 100 senators next week while Republican House of Representatives Speaker Kevin McCarthy vowed to investigate.

Democratic President Joe Biden said he was taking steps to tighten security. "While we are still determining the validity of those documents, I have directed our military and intelligence community to take steps to further secure and limit distribution of sensitive information," he said in a statement.

READ MORE:US air force employee in custody over classified Pentagon documents leak

More charges expected 

A criminal complaint made public on Friday charges Teixeira with one count of violating the Espionage Act related to the unlawful copying and transmitting of sensitive defence material, and a second charge related to the unlawful removal of defence material to an unauthorised location.

A conviction on the Espionage Act charge carries up to 10 years in prison.

The charges are connected to just one leaked document so far, a classified record that described the status of the Russia-Ukraine conflict and included details about troop movements on a particular date.

Experts expect more charges as investigators examine each leaked document, confirm its classification status and decide which can be shown to a jury without damaging national security.

Teixeira could also face more counts depending on the number of times he separately uploaded and transmitted each document.

"They are going to pick the ones [documents], I would imagine, that foreign governments have already seen," said Stephanie Siegmann, the former national security chief for the US Attorney's office in Boston and now a partner with the Hinckley Allen law firm.

In a sworn statement, an FBI agent said Teixeira had held a top secret security clearance since 2021 and also had sensitive compartmented access to other highly classified programs.

Since May 2022, the FBI said, Teixeira has been serving as an E-3/airman first class in the Air National Guard and has been stationed at Otis Air National Guard Base in Massachusetts.

Siegmann said one lingering question is why a 21-year-old National Guardsman held such a top-level security clearance.

"That's an issue that Department of Defense needs to now deal with," she said. "Why would he be entitled to these documents about the Russia-Ukrainian conflict?"

READ MORE: Pentagon warns of 'serious' security risk from leaked US documents