The US has denied that any of its unmanned spy planes have been shot down by Iran.
Pentagon spokesman Colonel Dave Lapan said there was no evidence to back up Iranian claims that it had shot down "Western spy drones".
Another US official told Reuters news agency that drones had in the past gone down in the Gulf, but that the cause had always been mechanical failure.
There has been no independent corroboration of the Iranian claim.
The head of the Revolutionary Guards' air force wing, Amir Ali Hajizadeh, said on Sunday that two "Western spy drones" had been shot down in the Gulf.
He said "many" other unmanned aerial vehicles (UAVs) had been shot down over an unspecified period but that this was the first time it had been reported.
But Col Lapan said there were "no recent reports that would corroborate what the Revolutionary Guard said about unmanned aerial vehicles".
The second official, speaking to Reuters on condition on anonymity, said one drone had crashed in the Gulf in early 2009 because of mechanical failings, but landed in international waters.
"We have had cases in which UAVs have gone down in the Gulf ... but I don't have any indication (of a case) where a UAV has been taken down by hostile fire in the Gulf," the official said.
The Revolutionary Guards were set up following the Islamic revolution in 1979, and its commanders have frequently delivered warnings to Israel.
Last August Iran unveiled what it said was its first domestically built drone, the Karrar.
It said it had a range of 1,000km (620 miles) and could carry two 250-pound (115kg) bombs, or a precision bomb of 500 pounds.
The Fifth Fleet of the US navy is based in Bahrain, on the other side of the Gulf from Iran.
Iranian commanders have threatened to block shipping through the strategic Strait of Hormuz if it is attacked.