A fourth US official said the documents appear to be legitimate.

The documents are attributed to the US Geospatial-Intelligence Agency and National Security Agency and note that Israel was still moving military assets in place to conduct a military strike in response to Iran's retaliatory ballistic missile attack on 1 October.

Iran's Islamic Revolutionary Guard Corps attack was in response to the Israeli assassinations of Hezbollah, Hamas, and IRGC leaders.

They were sharable within the “Five Eyes,” which are the US, Great Britain, Canada, New Zealand, and Australia.

The documents, marked top secret and dated 15 and 16 October, began circulating online Friday after being posted on Telegram by an account called “Middle East Spectator.” CNN was the first to report them.

They describe preparations Israel appears to be making for a strike against Iran. One of the documents says the plans involve Israel moving munitions around, CNN reported.

One of the documents suggests something that Israel has always declined to confirm publicly: that the country has nuclear weapons. The document says the US has not seen any indications that Israel plans to use a nuclear weapon against Iran, CNN said.

According to the report, "the US has observed the Israeli Air Force handling ALBMs (air-launched ballistic missiles) like 'Golden Horizon' (at least 16) and 'ROCKS' (at least 40) at Hatserim airbase since 8 October."

"From 15 to 16 October, the Israeli Air Force also handled several ASMs (air-to-surface missiles) at Ramot David and Ramon airbases, indicating an imminent attack," it said.

Additionally, the Israeli Air Force carried out "an LFE exercise on 15 October to practice air-to-air refuelling and combat search and rescue operations with a large number of aircraft, including at least three KC-707 tankers, one G-550 Gulfstream AWACs aircraft, and possibly fighter jets," the channel added from the leaked documents.

Another document says it is sourced to the National Security Agency and outlines Israeli air force exercises involving air-to-surface missiles, also believed to be in preparation for a strike on Iran. CNN is not quoting directly from or showing the documents.

One of the two documents resembled the style of other materials from the US National Geospatial-Intelligence Agency leaked by Jack Teixeira, an Air National Guardsman who pleaded guilty in March to leaking highly classified military documents about Russia’s war on Ukraine and other national security secrets.

The Telegram channel involved in the leak identifies itself as being based in Tehran, Iran's capital. It previously published memes featuring Iran’s Supreme Leader Ayatollah Ali Khamenei and material supporting the “Axis of Resistance” groups in the Middle East.

It self-identifies as "an open-source news aggregator."

"We are independent journalists and not tied to any government entity or organization," it declared.

Following the US reaction, the Telegram channel reiterated its rejection of claims made by various media outlets, including Axios and the Jerusalem Post, stating that it is "Iran-affiliated."

"We have no connection to the original source, which we assume to be a whistleblower within the U.S. Department of Defense," it affirmed in a press release.

The investigation is also examining how the documents were obtained — including whether it was an intentional leak by a member of the US intelligence community or obtained by another method, like a hack — and whether any other intelligence information was compromised, one of the officials said.

As part of that investigation, officials are working to determine who had access to the documents before they were posted, the official said.

"As far as we are aware, the documents first appeared in a private Telegram group with just over 7,000 members, where the leaker was likely present," the Telegram channel said.

In a statement, the Pentagon said it was aware of the reports of the documents but did not have further comment.