The US is investigating a leak of classified documents describing an American assessment of Israel’s plans to attack Iran, House of Representatives Speaker Mike Johnson has confirmed.

The documents were reportedly published online last week and are said to describe satellite imagery showing Israel moving military assets in preparation for a response to Iran’s missile attack on 1 October.

The documents, marked top secret, were shareable within the Five Eyes intelligence alliance of the US, Britain, Canada, New Zealand and Australia, CBS, the BBC’s US partner, reported.

For weeks Israel has been deciding how and when to respond to Iran’s latest missile attack. Israel’s defence minister has warned it will be “deadly, precise and surprising”.

The two documents reportedly appear to be attributed to the US National Geospatial Intelligence Agency and National Security Agency (NSA), and were published on an Iranian-aligned Telegram account on Friday.

Johnson, the highest-ranking member of Congress, told CNN on Sunday that “the leak is very concerning”.

“There’s some serious allegations being made, there’s an investigation under way, and I’ll get a briefing on that in a couple of hours,” the Louisiana Republican lawmaker said.

The Pentagon confirmed in a statement that it was aware of reports about the documents, but did not comment further.

The US agencies involved, as well as the Israeli government, have not publicly commented on the leak.

CNN and Axios first reported the leak, which confirms once again that the US spies on its close ally Israel.

One document makes a reference to Israel’s nuclear capabilities – which neither the US nor Israel ever officially acknowledge – apparently ruling out the use of such an option in any planned strike.

One former American intelligence official told the BBC the unauthorised release was probably an attempt to expose the scale of the planned retaliation, possibly to disrupt it.

The US is investigating whether the information was intentionally leaked by a US agent, or whether it was stolen, possibly through hacking, officials told the Associated Press (AP).

The two documents appear to be based on satellite information obtained from 15-16 October.

The first is titled: “Israel: Air Force Continues Preparations for Strike on Iran and Conducts a Second Large-Force Employment Exercise,” according to Reuters news agency. It describes ballistic and air-to-surface missile handling.

The second is titled: “Israel: Defense Forces Continue Key Munitions Preparations and Covert UAV Activity Almost Certainly for a Strike on Iran”. It discusses Israeli drone movements.

On Friday, US President Joe Biden said he had a “good understanding” of what Israel was planning.

“Do you have a good understanding of what Israel is going to do right now in response to Iran… and when they will actually respond?” a reporter asked him.

“Yes, and yes,” Biden replied.

“Can you tell us?” asked the reporter.

“No, and no.”