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.