WASHINGTON — A leak of classified US Defense Department documents is a “very serious” risk to national security, the Pentagon has said.

The documents appear to include sensitive information regarding the war in Ukraine, as well as on China and US allies.

Officials say the files are in a format similar to documents issued to senior leaders. An investigation has been opened to determine the source of the leak.

The documents — some of which officials believe have been altered — first appeared on online platforms such as Twitter, 4chan and Telegram, as well as on a Discord server for the video game “Minecraft”.

In addition to highly detailed information about the war in Ukraine, some of the leaked documents are said to cast light on sensitive briefing materials relating to US allies.

Other documents reportedly focus on defense and security issues in the Middle East as well as in the Indo-Pacific region.

Speaking to reporters on Monday, a high-ranking Pentagon official said the documents were “a very serious risk to national security and have the potential to spread disinformation”.

“We’re still investigating how this happened, as well as the scope of the issue,” said Chris Meagher, the assistant to the secretary of defense for public affairs.

“There have been steps to take a closer look at how this type of information is distributed and to whom.”

Meagher declined to answer when asked if the Pentagon believes the documents to be genuine, although he said that some “appear to have been altered”.

The Justice Department is now investigating the leak, alongside officials from the Pentagon, White House and elsewhere in the US government.

The format of the documents is similar to that “used to provide daily updates to our senior leaders on Ukraine and Russia-related operations, as well as other intelligence updates,” Meagher added.

The Pentagon first became aware of the document leak last week, with Defense Secretary Lloyd Austin first briefed on the matter on April 6, he said.

Meagher said that the document leak has prompted US officials to reassure its allies “of our commitment to safeguarding intelligence and fidelity to our security partnerships”.

At a separate briefing, national security spokesman John Kirby said that US President Joe Biden was first briefed about the leak last week.

When asked whether the leak has so far been contained and whether other documents have yet to be released, Kirby said “I don’t know”.

BBC News has so far reviewed more than 20 of the documents, many of which appear to detail the deployment and state of Ukrainian and Russian forces ahead of a long-awaited spring offensive by Ukrainian forces.

Some documents, for example, appear to outline US training and equipment being provided to Ukraine ahead of the offensive, as well as when various Ukrainian units will be ready and the anticipated delivery time of military supplies.

While Meagher declined to comment on the potential impact that the documents could have on the front lines in Ukraine, he said that “the Ukrainians have demonstrated their capability and competence in this war”.

“The president and secretary [of defense] have both made clear that the United States is going to be with them for as long as it takes,” he said. — BBC