A highly-classified US document revealing Israel's preparations for a strike on Iran has been leaked.

While the US have not denied their authenticity, a person familar with the matter confirmed that the documents were genuine, according to the New York Times. One US official told CNN that the leak is "deeply concerning".

The leaked documents expose Israeli military drills and weapons placement ahead of a planned retaliation attack against Iran. The documents, which analyse US satellite imagery, suggest a potential strike in the coming days.

The Israeli government had promised to hit back at Iran, following a barrage of Iranian missiles earlier this month, which was itself a response to an Israeli attack.

The documents, dated October 15 and 16, began circulating online on Friday after being posted on Telegram by a pro-Iranian account called 'Middle East Spectator'.

One document titled 'Israel: Air Force Continues Preparations for Strike on Iran' is tied to the National Security Agency.

It outlines Israeli air force exercises involving air-to-surface missiles in a rehearsal of a strike.

The rehearsals also involve long-range, air-launched ballistic missiles and air refueling planes.

The documents also reveal that Israel is conducting covert surveillance on Iran with drones.

The second document details how Israel is shifting the placement of its missiles and weapons in case Iran responds with strikes.

The US intelligence document also confirms that Israel possesses a nuclear weapon - something that Israel has always declined to confirm publicly.

The two documents have markings indicating they are meant to be seen only by the US and its 'Five Eyes' allies — Australia, Canada, New Zealand, and the UK.

The FBI, the Pentagon, and other US intelligence agencies are involved in the investigation into the leak.

Mick Mulroy, a retired CIA officer, said: “If it is true that Israeli tactical plans to respond to Iran’s attack on October 1 have been leaked, it is a serious breach."

He added: "The future coordination between the US and Israel could be challenged as well. Trust is a key component in the relationship, and depending on how this was leaked that trust could be eroded.”

There is relief among some US officials that the leak was not worse.

One official said: "These two documents are bad but not horrible. The concern is if there are more."

Meanwhile, on Friday, US President Joe Biden confirmed that he knew when Israel planned to strike and what kind of targets it had chosen

President Biden has previously called on Israel to avoid Iran’s nuclear sites and its energy facilities.