Washington (AFP) - US drones have been a key tool in conflicts against insurgent organizations such as the Taliban and the Islamic State group, but Iran's downing of one of the aircraft highlights their limitations against more sophisticated adversaries.

While drones offer the significant attraction of not putting American lives at risk and can stay aloft for more than a day, allowing for extended surveillance missions, they can be vulnerable to air defenses, are often expensive, and their loss can lead to sensitive hardware falling into the wrong hands.

American drones "are generally not stealthy; they are generally not aerodynamically impressive," said Michael O'Hanlon, a senior fellow and director of research in foreign policy at the Brookings Institution.

"I believe sophisticated air defenses will continue to have good chances to shoot down an aircraft like an RQ-4 whenever it is in position to do meaningful surveillance of their territories or other assets," he said, referring to the type of drone brought down by Iran last week near the strategic Strait of Hormuz.

But "it takes quite a bit to target an American military asset, and most countries won't do it just because Iran seems to have 'gotten away with it' here," O'Hanlon said, noting that punishing US sanctions on the country mean it is not emerging unscathed.

"These drones were not, generally speaking, designed with contested airspace in mind," said Arthur Holland Michel, co-director of the Center for the Study of the Drone at Bard College.

"If you looked at the operational history of the RQ-4 Global Hawk, you'll see that it generally avoids denied-airspace environments," he said.

The US military did not immediately respond to a request for comment on concerns over the vulnerability of drones to air defenses.

Holland Michel said that Predator drones had been shot down in the Balkans and Iraq, while a stealthy RQ-170 was lost over Iran in 2011, with Tehran claiming to have used it to build a copy of its own.

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"There are some possibilities for operating in contested airspace, but generally no, this is a system that if someone wants to shoot it down, they probably can," he said.

- 'Tremendously important' -

But American drones have proved "tremendously important" in "counter-terrorism and counter-insurgency style operations," Holland Michel said.

US forces have used drones to gather intelligence, monitor enemy forces and watch battles and raids unfold in real-time in countries such as Iraq and Afghanistan.

"They really showed their value in being able to track individual, moving targets persistently over extended periods of time," he said.

The loss of the US drone -- coupled with attacks on tanker ships the US has blamed on Iran -- has seen already-high tensions between the two countries spike further, raising fears of an unintended slide toward conflict that both sides have said they want to avoid.

US President Donald Trump ordered strikes on Iranian targets last week but said he called them off "10 minutes" before they were to have been launched, and Washington has now imposed sanctions on Iran's supreme leader Ayatollah Ali Khamenei and a string of military leaders.

In the event of a war, US drones might paradoxically be safer, O'Hanlon said.

"In wartime, they would do better, especially higher-altitude ones, because we would presumably jam or destroy enemy radars. But not in peacetime," he said.

The US also has stealth drones active in its inventory: the RQ-170 and RQ-180, Holland Michel said.

"If any US drones were to play a part against Iran in a contested environment, those would be the two most likely contenders," he said.

"They are designed with contested airspace in mind," Holland Michel said, but have not been put to the test "against a truly sophisticated adversary yet, at least as far as is publicly known."