The source of the leaked documents is unknown and as of Monday the Department of Defense was still working to assess their authenticity, though it has acknowledged they appear to contain sensitive, classified material.

The data has been present online for weeks or possibly even longer, though it only gained widespread media attention earlier this month. Here is what we know so far about the breach:

- What are the documents about? -

Many relate to the war in Ukraine. One provides information about the status of the conflict as of early March, including on Russian and Ukrainian casualties, while others detail the situations on specific fronts such as the battleground city of Bakhmut.

Another provides information about Ukrainian air defenses, which have been key to countering Russian missile and drone strikes, while one shows details on international efforts to build up Kyiv's military forces.

Other documents aren't related to Ukraine. Some, for example, point to US surveillance of its allies, such as one that states leaders of Israel's Mossad intelligence agency advocated for domestic protests against a controversial Israeli judicial reform plan that would have given lawmakers substantially more control over the supreme court.

- Are they genuine? -

The Pentagon says it is working to "assess the validity of the photographed documents that are circulating on social media sites," but admitted they "appear to contain sensitive and highly classified material."

At least one document seems to have been manipulated to make it say that Ukraine had suffered higher casualties than Russia, when the apparent original version said the reverse was true.

But US officials reportedly believe that many of the documents are genuine.

- What is US doing in response? -

The Justice Department has opened a criminal investigation into the apparent leak, while an assessment into its potential impact on national security is ongoing.

US officials have also been in touch with Washington's international allies about the issue, and relevant congressional committees have been informed.

- What impact could leak have? -

The impact of the leak could be significant, putting US intelligence sources at risk and also providing Russia with valuable information on the status of Ukrainian forces.

Documents pertaining to America's allies could also be a source of diplomatic embarrassment, detailing US surveillance of countries with which Washington has close ties.

- Where did documents appear? -

The documents were posted to various social media platforms and other sites, including Twitter, 4Chan and Discord.

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

Independent investigative outlet Bellingcat has looked into where the documents first appeared, reporting that some may have been online before January 2023.

Some were posted on Discord -- a popular chat platform for video gamers -- in channels for fans of a YouTube celebrity and players of the "Minecraft" computer game, Bellingcat said.