Tehran: Iran gave two warnings before downing a US reconnaissance drone over the Gulf of Oman, the commander of the aerospace arm of the country's elite Revolutionary Guards said on Friday.

"Twice we... sent warnings," Brigadier General Amirali Hajizadeh told the state television.

Hajizadeh said that even pilotless drones, like the one shot down on Thursday, had systems to relay warnings and other communications to their operators thousands of kilometres (miles) away in the United States.

"This aircraft possesses a system which allows it to relay the signals and information it receives to its own central system," he said.

"Unfortunately, when they failed to reply and the army gave a second appeal at 3:55 a.m. (2325 GMT Wednesday) and they kept on getting nearer and the aircraft made no change to its trajectory, at 4:05 a.m. (2335 GMT) we were obliged to shoot it down."

Hajizadeh stressed that the order was given only after the aircraft had entered Iranian airspace.

Thursday's downing — which Washington insists was above international waters — has seen tensions between the two countries spike further after a series of attacks on tankers the US has blamed on Iran.