Iran tried to shoot down an unmanned US surveillance drone in international airspace over the Gulf, the US claims, as tensions rise further between Tehran and Washington.

Iran fires at US drone in international airspace.

The drone was attacked last week but, US officals say, it was undamaged and returned safely to its base.

Pentagon spokesman George Little confirmed the MQ-1 "Predator" military drone was flying 16 nautical miles from Iran when it was intercepted by Russian-made Iranian SU-25 "Frogfoot" aircraft and was fired upon.

President Barack Obama and Defense Secretary Leon Panetta were both advised early on about the incident, which took place on 1 Nov.

Mr Little said Iran has since been warned the United States will keep conducting surveillance flights in the region.

"We have a wide range of options from diplomatic to military to protect our military assets and our forces ... and will do so when necessary," he said.

Asked whether the Iranians may have been firing warning shots, Little said the US assessment was that the Iranian aircraft were aiming to shoot down the US drone.

"Our aircraft was never in Iranian air space. It was always flying in international air space. The internationally recognized territorial limit is 12 nautical miles off the coast and we never entered the 12 nautical mile limit," Little said.

The incident came a year after a CIA drone crashed in Iran - giving the Islamic Republic access to sensitive American technology.

In December 2011 Iranian TV broadcast pictures of the US surveillance drone it claimed had been shot down (see video below). US officials denied this, saying it had malfunctioned.