The online leaks of scores of highly classified documents about the Ukraine war present a "very serious" risk to national security, and senior leaders are quickly taking steps to mitigate the damage, a top Pentagon spokesman said Monday, according to The Associated Press.

Chris Meagher, assistant to the secretary of defense for public affairs, told reporters that Defense Secretary Lloyd Austin first became aware on Thursday that a number of classified briefing slides detailing the US military efforts in the Ukraine war and intelligence involving other nations were leaked.

In the days since Austin was notified, he has reached out to allies, held daily meetings to assess damage and set up a group not only to assess the scope of the information lost but review who has access to those briefings.

The department is looking closely at "how this type of information is distributed and to whom," Meagher said, but would not say if steps had already been taken to tighten control over who can access it.

At the State Department, spokesman Vedant Patel told reporters that US officials "are engaging with allies and partners at high levels over this, including to reassure them of our commitment to safeguarding intelligence and the fidelity of securing our partnerships."

One of the leaked documents includes an assessment by US officials that the leadership of the Israeli Mossad encouraged the protests against the judicial reform.

Another document outlines scenarios in which Israel would agree to provide “lethal aid” to Ukraine.

On Sunday, Western security experts and US officials quoted by Reuters said they suspect that the highly classified military and intelligence documents that appeared online could have been leaked by someone from the United States.

US officials told Reuters said the investigation is in its early stages and those running it have not ruled out the possibility that pro-Russian elements were behind the leak, which is seen as one of the most serious security breaches since more than 700,000 documents, videos and diplomatic cables appeared on the WikiLeaks website in 2013.