US President Joe Biden expressed his "concern" Thursday about the leaking of classified US documents, but without seeing what happened as an imminent danger.

"I am concerned that this will happen," Biden told reporters after his meeting with Irish President Michael Higgins in Dublin.

He added that the "investigation is ongoing" and appears to be close to conclusions.

The US Justice Department opened a criminal investigation after documents leaked online detailing Washington's views on the war in Ukraine and implying that the United States was collecting intelligence on close allies.

The Pentagon considered the leak a "very serious" threat to the national security of the United States.

The US authorities have not yet publicly confirmed the authenticity of these images, which are circulated on various websites on the Internet, and their authenticity has not been independently verified.

The Washington Post reported that the leak was behind a young man who worked at a military base and published its information through a private group on the Internet.

And the young man, who called himself "OG" (OG) for months, published hundreds of pages of these documents after he transferred them from files at the military base where he works, according to the newspaper.

The documents published on the Internet reveal the concern of US intelligence about the ability of the Ukrainian army to launch a counterattack against the Russian forces, due to problems in training and supply.

A document seen by Agence France-Presse speaks of US concerns regarding Ukraine's ability to continue to defend itself against Russian strikes.