The FBI is investigating the public disclosure of a pair of highly classified intelligence documents describing Israel's preparations for a retaliatory strike on Iran, the bureau said on Tuesday.

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

White House national security spokesman John Kirby said on Monday that it has not been determined whether the disclosure was a hack or a leak. President Joe Biden was watching the results of the investigation closely, he added.

"We're not exactly sure how these documents found their way into the public domain," Kirby told reporters.

"The president remains 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.

The documents appear to have been prepared by the National Geospatial-Intelligence Agency, describing U.S. interpretations of Israeli Air Force and Navy planning based on satellite imagery from Oct. 15-16.

They began circulating last week on the Telegram messaging app. Israel has been planning a response to a ballistic-missile barrage carried out by Iran on Oct. 1, Tehran's second direct attack on Israel in six months. Israel has intensified its offensive in Gaza and Lebanon, days after the killing of Hamas leader Yahya Sinwar.

U.S. House of Representatives Speaker Mike Johnson on Sunday said an investigation was under way.