The US is investigating an unauthorised release of classified documents that assess Israel's plans to attack Iran, three US officials told The Associated Press. 

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 continues to move military assets in place to conduct a military strike in response to Iran's blistering ballistic missile attack on October 1. 

They were shareable within the "Five Eyes," which are the US, Great Britain, Canada, New Zealand and Australia.

The documents, which are marked top secret, were posted online to Telegram and first reported by CNN and Axios. 

The officials spoke on the condition of anonymity because they were not authorised to discuss the matter publicly.

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. 

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

The documents emerged as the US has urged Israel to take advantage of its elimination of Hamas leader Yahya Sinwar and press for a ceasefire in Gaza, and has likewise urgently cautioned Israel not to further expand military operations in the north in Lebanon and risk a wider regional war. 

However, Israel's leadership has repeatedly stressed it will not let Iran's missile attack go unanswered.

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

The US has been bolstering Israel's air defence system.

It has provided Israel with one of its most advanced air defence systems, the Terminal High-Altitude Area Defense (THAAD) battery, to help defend against further Iranian ballistic missile attacks.

The Pentagon has also sent about 100 troops to operate it in Israel. 

AP/ABC