Iranian warplanes fired at an unarmed US drone in international airspace last week but did not hit the aircraft, the Pentagon said on Thursday, disclosing details of an unprecedented incident that triggered a formal warning to Teheran through diplomatic channels.

The Nov 1 intercept was the first time Teheran had fired at an unmanned US aircraft, in a stark reminder of how tensions between the United States and Iran could escalate quickly into violence.

If Iran had hit the drone, as the Pentagon believes it was trying to do, it could have forced US retaliation - with the potential consequences that entails.

According to the timeline provided by the Pentagon, two Iranian SU-25 "Frogfoot" aircraft intercepted the US drone at about 4:50 am as it conducted a routine, but classified, surveillance mission over Gulf waters about 16 nautical miles off the Iranian coast.

Pentagon spokesman George Little said the aircraft fired multiple rounds at the Predator drone and followed it for at least several miles as it moved farther away from Iranian airspace.

"We believe that they fired at least twice and made at least two passes," he said.

International airspace begins after 12 nautical miles and Little said the drone at no point entered Iranian airspace. Last year, a crashed CIA drone was recovered inside Iran.

Defense Secretary Leon Panetta was quickly notified of the incident, as were members of Congress and the White House, Little added. The US also sent Iran a warning through diplomatic channels, saying it would defend its military assets and would keep sending aircraft on such surveillance operations.

"There is absolutely no precedence for this," Little said. "This is the first time that a (drone) has been fired upon to our knowledge by Iranian aircraft."

Iran's defense minister confirmed on Friday that two Iranian fighter jets fired on an unarmed US Predator drone in the Gulf last week.

"Last week an anonymous plane entered the space over the territorial waters of the Islamic Republic of Iran in the Persian Gulf area which, due to the timely, clever and decisive action of our armed forces, was forced to flee," Ahmad Vahidi was quoted on ISNA as saying.

Reuters