Washington, April 11 : A major leak of classified US Department of Defence documents poses a major risk to the country's national security, with President Joe Biden's administration scrambling to assess and contain the fallout from the development, according to the Pentagon. Alongside officials from the Pentagon and White House, the Justice Department is currently investigating how the trove of highly sensitive documents, which include details about how the US spies on friends and foes as well as intelligence on the Russia-Ukraine war, ended up on social media sites, reports the CNN. Addressing reporters on Monday, Chris Meagher, Assistant to the Secretary of Defence for Public Affairs, said the documents were "a very serious risk to national security and have the potential to spread disinformation", the BBC reported. "We're still investigating how this happened, as well as the scope of the issue," he said, adding that the Pentagon is reassessing their process as to who gets access to such sensitive documents. "There have been steps to take a closer look at how this type of information is distributed and to whom." Meagher however, 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 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", the BBC quoted the top Pentagon official as saying. The Pentagon first became aware of the document leak last week, with Defence Secretary Lloyd Austin first briefed on the matter on April 6, he said. Meagher added that the document leak has prompted US officials to reassure its allies "of our commitment to safeguarding intelligence and fidelity to our security partnerships". The documents -- some of which officials say may 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. A source close to Ukrainian President Volodymyr Zelensky told CNN that Kiev has already altered some of its military plans because of the leak. Other documents reportedly focus on defence and security issues in the Middle East as well as in the Indo-Pacific region. Meanwhile, Congressional lawmakers have expressed concerns about the apparent scope of the leak and sensitivity of the documents posted online but largely remain in the dark about what has occurred. Both House and Senate Intelligence Committee leaders are demanding answers from the Biden administration. House Intelligence Chairman Representative Mike Turner is scheduled to receive a briefing and his Senate counterparts have jointly requested one as well, reports CNN. At a separate briefing also on Monday, national security spokesman John Kirby said that President Biden had been 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." /IANS