President Joe Biden has insisted he is 'not concerned' about the leak of top secret Pentagon documents and authorities are 'getting close' to tracking down the leaker who posted the files on Discord.

The president broke his silence on the massive security breach on Thursday and downplayed the significance by suggesting the information on the Ukraine war and US intelligence operations around the world were out of date.

His first comments followed reports overnight that the man behind the most significant leak of classified documents in a decade is a 20-something video game and gun enthusiast who worked at a secure military site.

Two members of the chat room, known as 'Thug Shaker Central', told the Washington Post the suspect shared the sensitive content with like minded-people.

'I'm concerned that it happened. But there's nothing contemporaneous that I'm aware of,' Biden said, when asked if he was concerned about the leak.

Experts have warned the breach could jeopardize overseas intelligence sources, damage relations with allies and even hinder Ukraine's war with Russia.

His remarks on the ultra-serious security breach came as he strolled through gardens at the official residence of Irish President Michael Higgins on the first bright and sunny day of his trip here.