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

Officials say the breadth of topics addressed in the documents, which touch on the war in Ukraine, China, the Middle East and Africa, suggest they were leaked by an American rather than an ally, the report said.

"The focus now is on this being a US leak, as many of the documents were only in US hands," Michael Mulroy, a former senior Pentagon official, told Reuters in an interview.

US officials 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.

The Russian embassy in Washington and the Kremlin did not respond to requests for comment.

Two US officials told Reuters on Sunday that they have not ruled out that the documents may have been doctored to mislead investigators as to their origin or to disseminate false information that may harm US security interests.

The White House referred questions to the Pentagon. The Pentagon reiterated it was reviewing the matter and had made a formal referral to the Department of Justice asking it to investigate the incident.

The documents which were leaked include an assessment by US officials that the leadership of the Israeli Mossad encouraged the protests against the judicial reform.

Following the publication, Prime Minister Benjamin Netanyahu spoke on Sunday with the head of the Mossad David Barnea.

Channel 12 News reported that Netanyahu asked Barnea to respond to the publication, and he explained that it was a wrong analysis on the part of US intelligence officials. The Mossad estimates that the analysis is a result of the approval given by Barnea to lower-ranking Mossad employees to join the protests against the reform.

Netanyahu accepted the explanation provided by Barnea. After that conversation, the Prime Minister's Office issued a statement on behalf of the Mossad, denying the report and clarifying that the it was "fundamentally false and unfounded."