Washington DC - The FBI and Pentagon are investigating a leak of classified US intelligence about Israel's plans for a retaliatory strike on Iran.

"The FBI is investigating the alleged leak of classified documents and working closely with our partners in the Department of Defense and Intelligence Community," the FBI said in a statement on Tuesday.

"As this is an ongoing investigation, we have no further comment."

The classified documents involve satellite imagery analysis from the US National Geospatial-Intelligence Agency and the US National Security Agency, according to press reports.

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They reportedly describe Israeli preparations for a retaliatory strike on Iran for its October 1 missile barrage on Israel – but do not identify any actual targets.

They were circulated on the Telegram app last week by an account called Middle East Spectator.

White House national security spokesman John Kirby, speaking to reporters on Monday, said US authorities do not know if "it was a leak or a hack at this point."

"We're not exactly sure how these documents found their way into the public domain," Kirby said. "I know the Department of Defense is investigating this, and I'm sure that as they work through that, they'll try to determine the manner in which they did become public."

Kirby said President Joe Biden is "deeply concerned about any leakage of classified information into the public domain."

"That is not supposed to happen, and it's unacceptable when it does," he said.