Merrick Garland speaks at the Department of Justice in Washington on Thursday. AP

US Attorney General Merrick Garland said the FBI arrested the man, Jack Teixeira, "in connection with an investigation into alleged unauthorised removal, retention, and transmission of classified national defence information."

The FBI said its agents had made an arrest and were conducting "authorised law enforcement activity at a residence in North Dighton, Massachusetts."
Video images played on news channels showed heavily armed officers in Massachusetts accompanying a young man wearing a gray t-shirt and bright red shorts into a waiting car. His head was bowed and his hair was close-cropped.

The leaks were a "deliberate, criminal act," the Pentagon said. Pentagon spokesman Brigadier General Pat Ryder referred questions about the investigation to the Department of Justice.

Some of the most sensitive leaked details are purportedly related to Ukraine's military capabilities and shortcomings, and information about US allies including Israel, South Korea and Turkey.

Police block a road in North Dighton, Massachusetts, on Thursday. AP

President Joe Biden earlier on Thursday said investigators were closing in on the source of the online leaks in what is believed to be the most serious security breach in years.

The New York Times earlier reported that Teixeira was a National Guardsman who led Thug Shaker Central, an online group where about 20 to 30 people shared their love of guns, racist memes and video games. The Times cited interviews and documents it reviewed.

Reuters