The breach appears to include assessments and secret intelligence reports that touch not only on Ukraine and Russia but also highly sensitive analyses of US allies.

"We have been in communication with the Department of Defense related to this matter and have begun an investigation," a Justice Department spokesperson told AFP.

A steady drip of dozens of leaked documents and slides have made their way onto Twitter, Telegram, Discord and other social media and chat sites in recent days, and new documents continue to surface.

The Pentagon said Friday it was "actively reviewing the matter" and that it had formally referred the apparent breach to the Justice Department.

US officials told the Washington Post that some documents appeared to be manipulated but many were consistent with CIA World Intelligence Review reports that are shared at high levels within the White House, Pentagon and State Department.

Defense analysts say any breach of internal classified US documents would be both damaging and potentially embarrassing.

In addition, the leak would prove valuable to Moscow by showing how deep US intelligence has penetrated parts of the Russian military apparatus, US media said.

Other documents include apparent information about internal debate within the governments of US allies.

Among the documents, for example, were discussions about South Korea's debate on whether to provide the United States artillery shells for use in Ukraine, The New York Times said.