The breach, which is being investigated by the Justice Department, appears to include assessments and secret intelligence reports that touch not only on Ukraine and Russia but also highly sensitive analyses of US allies.

The documents circulating online pose "a very serious risk to national security and have the potential to spread disinformation," Chris Meagher, the assistant to the secretary of defense for public affairs, told journalists.

"We're still investigating how this happened, as well as the scope of the issue. There have been steps to take a closer look at how this type of information is distributed and to whom," Meagher said.

A steady drip of dozens of photographs of documents have been found on Twitter, Telegram, Discord and other sites in recent days, though some may have circulated online for weeks, if not months, before they began to receive media attention last week.

Meagher declined to comment on whether the documents are genuine, saying a Pentagon team is working to make that assessment, but noted that photos circulating online seemed to show sensitive information.

"Photos appear to show documents similar in format to those used to provide daily updates to our senior leaders on Ukraine and Russia-related operations, as well as other intelligence updates," he said, but some "appear to have been altered."

This includes a document circulating online that seems to have been altered to make it show that Ukraine had suffered higher casualties than Russia, when the apparent original version said the reverse was true.