US probes leak of documents

US officials are trying to identify who leaked dozens of classified Pentagon documents that appeared online with details that include secret assessments of Ukraine's combat power and intelligence gathering on America's allies, including South Korea, Canada and Israel.

Security analysts who have reviewed the documents on social media sites told various US media that the documents also include sensitive briefing material on China, the Indo-Pacific military theater and the Middle East.

Chris Meagher, the assistant to the secretary of defense for public affairs, said Monday at a briefing that the Pentagon first became aware of the document leak last week, with Defense Secretary Lloyd Austin first briefed on the matter on April 6.

At a separate briefing, national security spokesman John Kirby said that President Joe Biden was first briefed about the leak last week.

Meagher said that the document leak has prompted US officials to reassure its allies "of our commitment to safeguarding intelligence and fidelity to our security partnerships".

NBC News reported Saturday that a senior US official said that the government's "working theory" is that the documents are real, although some of them could have been altered.

It remains unclear whether the leak is the result of a hack by a foreign adversary or whether the disclosures came from within the US government or through a US ally with access to American intelligence reporting.

The evidence that this is a leak, and not a hack, appears strong, said The New York Times.

The leak is expected to raise questions among US allies about whether Washington can be trusted with secret information, said security experts.

A South Korean senior government official on Monday in a briefing cast doubt on the legitimacy of the documents, saying there are suggestions that some of the information might be fabricated. He said that his country will demand that the US take "appropriate measures" once the investigation has concluded.

The documents first appeared last month on social media websites, beginning with the Discord server for the video game Minecraft and 4Chan. Their existence was first reported on Friday by the Times.

A Pentagon official said that the Defense Department is assessing the "validity" of the documents, and an interagency effort is assessing how the leak could affect national security and relationships with allies.

"Over the weekend, US officials have engaged with Allies and partners and have informed relevant congressional committees of jurisdiction about the disclosure," said Pentagon deputy press secretary Sabrina Singh in a statement.

Two US officials told Reuters on Sunday that they haven't 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.

Some experts say it could be an American who leaked the documents. Michael Mulroy, a former senior Pentagon official, told Reuters in an interview that the focus now is on this being a US leak, as many of the documents "were only in US hands".

But those running the investigation said they haven't ruled out the possibility that pro-Russian elements were behind the leak.

CNN said it has reviewed 53 leaked documents, all of which appear to have been produced between mid-February and early March.

They provide a rare window into how the US spies on allies and adversaries alike, CNN said.

While US allies are aware that the US intelligence community collects information on friendly nations, diplomats from some of the countries mentioned told CNN it was frustrating — and harmful to the US reputation — to see that information exposed publicly.

One of the documents, dated Feb 23 and marked "Secret", outlines in detail how Ukraine's S-300 air defense systems would be depleted by May 2 at the current usage rate, according to NBC News.

In a statement to the Times, Mykhailo Podolyak, an adviser to Ukraine's president, Volodymyr Zelensky, said the documents were filled with "fictional information".

"There is not the smallest doubt that this is yet another element of hybrid warfare," he said. "Russia is trying to influence Ukrainian society, sow fear, panic, mistrust and doubt. It's typical behavior."

But Ukraine has already altered some of its military plans because of the leak, a source close to Zelensky told CNN.

Agencies contributed to this story.