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The US is investigating a leak of top-secret American intelligence about Israel’s plans for retaliation against Iran, according to three people familiar with the matter. An acquaintance confirmed the authenticity of the documents.

The leak is “very concerning,” a US official told CNN.

The documents, dated October 15 and 16, began circulating online on Friday after being posted on Telegram by an account called ‘Middle East Spectator’.

They are marked top secret and have markings indicating they are intended to be seen only by the US and its “Five Eyes” allies – Australia, Canada, New Zealand and the United Kingdom.

They describe the preparations that Israel appears to be making for an attack on Iran. One of the documents, which says it was compiled by the National Geospatial-Intelligence Agency, says the plans involve moving munitions through Israel.

Another document says it comes from the National Security Agency and outlines Israeli air force exercises with air-to-surface missiles, also believed to be in preparation for an attack on Iran. CNN does not quote directly from or show the documents.

A US official said the investigation is looking into who had access to the alleged Pentagon document. Such a leak would automatically trigger an investigation by the FBI, along with the Pentagon and US intelligence services. The FBI declined to comment.

The leak comes at a particularly sensitive time in US-Israeli relations and will undoubtedly anger Israelis, who are preparing to attack Iran in response to Iranian missile fire on October 1. One of the documents also suggests something that Israel has always rejected. to publicly confirm: that the country has nuclear weapons. The document says the US has seen no indication that Israel plans to use a nuclear weapon against Iran.

“If it is true that Israeli tactical plans to respond to Iran’s attack on October 1 were leaked, that is a serious breach,” said Mick Mulroy, former deputy assistant secretary of defense for the Middle East and a retired CIA -officer.

Mulroy added that “future coordination between the US and Israel could also be tested. Trust is a key component of the relationship, and depending on how this is leaked, that trust could be eroded.”

The National Security Council referred CNN to the Office of the Director of National Intelligence and the Pentagon for comment. The Pentagon and the National Geospatial-Intelligence Agency declined to comment. CNN has contacted the National Security Agency for comment.

Another US official said that “these two documents are bad, but not terrible. The concern is whether there are more.”

It is not clear how the documents became public, nor whether they were hacked or deliberately leaked. The US is already on high alert for Iranian hacking campaigns. US intelligence agencies said in August that Iran had hacked documents belonging to Donald Trump’s campaign.

Axios first reported on the leaked documents on Saturday.

A major U.S. intelligence leak last year also strained U.S. relations with allies and partners, including South Korea and Ukraine, after a 21-year-old Air National Guardsman posted highly classified information on the social media platform Discord.

This story has been updated with additional information.

CNN’s Evan Perez and Katie Bo Lillis contributed to this report.