The Global Hawk, an advanced US navy surveillance drone, was flying at high altitude -- it can reach 60,000 feet (18 kilometres) -- early Friday local time when it was struck by a ground-to-air missile by Iran's elite Revolutionary Guards.

"The shooting down of the drone shows Iran is revealing a capability and choosing to message it to the United States," said Becca Wasser, an analyst at Rand Corp.

"The fact that Iran was able to shoot down the drone demonstrates that they have developed or purchased fairly significant capabilities and are skilled at employing these systems."

Wasser noted an Iranian claim that it had used a home-made surface-to-air missile (SAM) system to shoot down the drone.

"If true, this would be significant because it is a domestically produced capability that Iran could replicate and potentially provide to proxy groups throughout the region to threaten US and partner militaries," she told AFP.

According to the Military Balance -- an annual publication by the International Institute for Strategic Studies (IISS) -- Iran has 32 batteries of Russian-made S-300 ground-to-air missiles that have been delivered by Moscow since 2016. They are seen as posing a serious threat.

The Islamic Republic has also developed Iranian versions of these missile systems, including the Bavar 373, SAM Tabas and SAM Raad which are regularly displayed at military parades.

The Revolutionary Guards claim that they shot down the US drone with a Khordad 3 missile, a version of the SAM Raad.

Senior Iranian officials and military officers have welcomed the strike and warned against a possible US retaliation.

"The enemy dispatched its most advanced, smartest and most sophisticated surveillance and reconnaissance aircraft to the forbidden area and everyone saw the shooting down of this unmanned aircraft," Iran's navy commander Rear Admiral Hossein Khanzadi said on Monday.

"I say confidently that this crushing response can be repeated and the enemy knows this," he said.

- Spy Plane -

The former head of a French intelligence service, who asked to remain anonymous, said that if the US airforce sends large numbers of aircraft over Iran, they should be prepared for some losses, because Iranian forces will be prepared.

"That said, in the case of the drone, it may not have had sufficient counter-measures -- to deceive, deflect or outmaneuvre the missile -- and the Americans thought that the Iranians would not dare" shoot it down, he told AFP.

Dan Gettinger, co-director at the Center for the Study of the Drone at Bard College, New York, said the Global Hawk is a large plane that flies slowly.

The plane "is not a stealthy aircraft -- it's very large" with a wingspan of 40 metres, similar to a Boeing 737, Gettinger said.

The drone's fate is "definitely going to be taken into account in future operations," Gettinger told AFP.

He said the incident is also reminiscent of the time the Soviet Union shot down a U-2 spy plane in 1960.

That event, targeting a piloted craft, "is a large part of the reason why the US started developing drones in the first place," Gettinger said.

Experts however say that the latest incident does not mean Iran is capable of building a firewall against the airforce of the world's largest military, whose superior firepower and electronic warfare capabilities are overwhelming.

At the end of 2015, just after Moscow agreed to sell the S-300 systems to Tehran, a move Israel tried in vain to prevent, the head of the Israeli Air Force General Amir Eshel said the missiles posed "a significant but not insurmountable challenge".

Douglas Barrie, senior fellow for military aerospace at IISS, said Iran had shown it is capable of shooting down a large drone.

But, he said, "this is a long way from being able to sustain air defence operations in the face of a significant attack involving missile strikes and counter-measures to degrade its air defences."