The breach is being investigated by the Justice Department and appears to include secret information on the war in Ukraine as well as sensitive analyses of US allies, whom American officials are now seeking to reassure.

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.

The documents circulating online pose "a very serious risk to national security and have the potential to spread disinformation," said Chris Meagher, assistant to the secretary of defense for public affairs, without confirming their authenticity.

"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 told journalists.

Many of the documents are no longer available on the sites where they first appeared, and the United States is reportedly continuing to work to have them removed.

Meagher said US Defense Secretary Lloyd Austin was not initially briefed on the issue until the morning of April 6 -- the day a New York Times story on the documents was published.

White House national security spokesman John Kirby told journalists that President Joe Biden was briefed on the leak "late last week," but did not provide a specific date.

Many of the documents relate to Ukraine, such as one that provides information on the country's air defenses or another on international efforts to build up its military forces.

- Reassuring allies -

But other documents point to US surveillance of its allies, such as one that states that leaders of Israel's Mossad intelligence agency advocated for domestic protests against a controversial judicial reform plan.

US officials have been in touch with Washington's allies over the issue, the State Department said.

They "are engaging with allies and partners at high levels over this, including to reassure them on our commitment to safeguarding intelligence and ability to secure our partnerships," spokesperson Vedant Patel told journalists.

The Pentagon also said relevant congressional committees have been informed.

Meagher said a Pentagon team is working to determine if the documents are genuine, but noted that images 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 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.

Kirby said the US government is worried that there may be more documents to come.

"We don't know who's responsible for this. And we don't know if they have more that they intend to post," Kirby said, adding: "Is that a matter of concern to us? You're darn right it is."

The fallout from the apparent leak could be significant -- even deadly -- potentially putting US intelligence sources at risk while giving the country's foes valuable information.

"Disclosure of sensitive classified material can have tremendous implications not only for our national security, but could lead to people losing their lives," Meagher said.