"Top secret" Washington is searching for a source of leaking military documents on the Internet

US officials are working to determine the source of a leak of highly classified military and intelligence documents that has spread over the Internet. Officials say the wide range of subject matter in the documents indicates they were leaked from a US citizen rather than an ally.

US officials are making strenuous efforts to locate the source of the leak of top-secret military and intelligence documents that have spread on the Internet, including details of Ukrainian air defenses and the Israeli intelligence service (Mossad).

Western security experts and US officials said they suspected someone from the United States might be behind the leak.

Officials say the wide range of topics in the documents, which cover the Ukraine, China, Middle East and Africa wars, indicate they were leaked from a US citizen rather than an ally.

"The focus now is that this is a leak from the United States because a number of these documents were only in the possession of the United States," said Michael Mulroy, a former senior Pentagon official.

US officials said the investigation was in its early stages and those in charge of his administration did not rule out the possibility that pro-Russian elements were behind the leak, which is seen as one of the most serious security breaches since the WikiLeaks leaks in 2013 that included more than 700,000 documents, videos and diplomatic cables.

Reuters says it has reviewed more than 50 documents titled "secret" and "top secret", after the leak was revealed, which first appeared last month on social media, starting with the Discord and Fortune platforms.

Although some of those documents were published weeks ago, the New York Times was first to report on them on Friday.

Some of these documents contained battlefield casualty estimates from Ukraine, but they appear to have been modified to reduce Russian losses. It is also not yet clear why at least one of these documents was marked "unclassified", even though it contained highly classified information. Some of the documents are marked "Novoren", which means they cannot be released to outsiders.

Two US officials said Sunday that they did not rule out the possibility of the documents being tampered with to mislead investigators about their provenance or to spread false information that could harm US security interests.

One of the documents, dated February 23 and marked "Confidential", details how the Ukrainian S-300 air defense systems will be depleted by May 2 at their current rate of use.

Such closely guarded information could be of great use to Russian forces, and Ukraine said its president and top security officials met on Friday to discuss ways to prevent the leaks.

Allied control

Another document, stamped "Top Secret" and taken from a CIA brief dated March 1, says that the Mossad, the Israeli secret service, supported protests against Israeli Prime Minister Benjamin Netanyahu's plans to take control of the Supreme Court.

The document said the United States learned of this through intelligence signals, indicating that Washington was spying on one of its most important allies in the Middle East.

In a statement Sunday, Netanyahu's office said the document was "completely false and baseless."

Another document detailed private discussions among senior South Korean officials about US pressure on the Asian ally to help supply arms to Ukraine and Seoul's policy not to do so.

An official at South Korea's presidential palace said Sunday that Seoul is aware of media reports about the leaked documents and that it intends to discuss the issues raised by the leaks with the United States.

The Pentagon did not address the content of these documents, including apparently allied surveillance.

Two US officials, speaking on condition of anonymity, said that while there was concern about the leaked documents at the Pentagon and intelligence agencies, the documents provided only a partial snapshot of the war in Ukraine going back a month, rather than recent assessments.

The officials added that intelligence agencies and the military are looking into their operations to see how much intelligence is shared internally.

The White House referred the matter to the Pentagon, which confirmed Sunday that it is under study and that it has formally referred it to the Department of Justice and asked it to investigate.

On Friday, the US Department of Justice said it was in contact with the Pentagon and had launched an investigation into the leaked documents, and declined to comment further.