President Biden is "deeply concerned" by the possible leak of U.S. intelligence documents regarding Israeli preparations for an attack on Iran, which the Pentagon is investigating, National Security Council spokesman John Kirby said Monday.

Kirby told reporters the Biden administration can't yet say if the classified information that became public over the weekend on the messaging app Telegram was leaked or the result of a hack, but the president will be "actively monitoring" the progress of the investigative efforts.

"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. "We're deeply concerned, and the president remains deeply concerned, about any leakage of classified information in the public domain. That is not supposed to happen, and it's unacceptable when it does."

White House press secretary Karine Jean-Pierre said the matter is "being looked into" and the president has "complete confidence" in the relevant federal agencies.

A defense official told CBS News over the weekend that the U.S. is looking into reports of the publication of what appear to be two highly classified intelligence documents with information about Israel's plans to respond to a missile attack by Iran on Oct. 1. House Speaker Mike Johnson also told CNN on Sunday that an investigation is underway. Kirby's remarks are the first on the record comments from a Biden administration official confirming the investigation.

The documents surfaced under a Telegram account called the Middle East Spectator. The channel has a history of posting pro-Iran content, although the account says it is not a pro-Iran group. The documents appear to show the planning of an Israeli retaliation on Iran, including the positioning and repositioning of weapons. The documents, marked top secret, were first reported Saturday by CNN and Axios.

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