The Kremlin has criticized what it called a "general tendency" to blame Russia for things when asked about initial accusations that Moscow was behind a major leak of U.S. intelligence documents.

U.S. national security officials have been trying to deal with fallout from the release of dozens of secret documents, including the impact of sensitive information from other U.S.-allied governments including Ukraine going public. 

The classified documents detail information ranging from Ukraine's air defenses to Israel's Mossad spy agency, with some U.S. experts saying it could be an American that leaked the intelligence. 

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

However, a former CIA officer said it was highly likely that Moscow organized the leak in order to spread confusion and potential divisions between Washington and its allies.

When asked about the allegations, Kremlin spokesperson Dmitry Peskov said, "I cannot comment on this in any way. You and I know that there is in fact a tendency to always blame everything on Russia. It is, in general, a disease."

Asked about whether Washington had spied on Ukrainian President Volodymyr Zelenskyy, Peskov, who called the leaks "quite interesting", said that could not be ruled out.

"But the fact that the U.S. has been spying on various heads of state, especially in European capitals, for a long time now, has come up repeatedly, causing various scandalous situations," he said.

Ukraine said its president and top security officials had met on Friday to discuss ways to prevent such leaks.

U.S. intel on Israel, South Korea

U.S. officials said the investigation, which is in its early stages, comes after one of the most serious security breaches since more than 700,000 documents, videos and diplomatic cables were published on the WikiLeaks website in 2013.

Some of the documents were posted online weeks ago, but their existence was first reported on Friday by the New York Times. Some of the documents allegedly give battlefield casualty estimates from Ukraine that appeared to have been altered to minimize Russian losses.

The Pentagon said on Sunday it was reviewing the validity of the documents that "appear to contain sensitive and highly classified material."

One document, marked "Top Secret" and from a CIA Intel update from March 1, says Israel's Mossad intelligence agency was encouraging protests against Prime Minister Benjamin Netanyahu's plans to tighten controls on the Supreme Court.

The document said the U.S. discovered the alleged information through signals intelligence, suggesting the U.S. had been spying on one of its most important allies in the Middle East. In a statement on Sunday, Netanyahu's office described the assertion as "mendacious and without any foundation whatsoever."

Another document reported on internal discussions among senior South Korean officials about U.S. pressure on Seoul to help supply weapons to Ukraine.

Two U.S. officials told Reuters under anonymity that Washington's military and intelligence agencies were looking at how widely some of their intelligence was being shared internally.

One of the officials added that investigators were looking at four or five theories why a U.S. citizen might have leaked the documents, from an angry employee to an insider who purposefully wanted to hurt U.S. national security.

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