Iran has fired at a US drone whilst it was in international airspace, the Pentagon announced.

In the first ever acknowledgement of such action, a Pentagon spokesperson confirmed an Iranian SU-25 military jet fired on a U.S. Predator drone on November 1, whilst the craft was conducting a routine surveillance operation in the Persian Gulf.

The unmanned craft was 16 nautical miles from the Iranian coast and was not hit, the Pentagon press secretary said.

The operation was "classified" he added, saying details were only being revealed now because the incident had leaked to the press earlier.

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Spokesman George Little said the incident was unprecedented, telling reporters: "Our aircraft was never in Iranian air space. It was always flying in international air space.

"The United States has communicated to the Iranians that we will continue to conduct surveillance flights over international waters over the Arabian Gulf, consistent with longstanding practice and our commitment to the security of the region."

Iran claimed to have shot down a U.S. drone last December, however the Pentagon claimed the drone crashed.