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.

Despite this, net use among this group is growing with the numbers trying the net doubling since 2011, it found.
It also revealed that 25% of disabled people have yet to use the net - a figure campaigners called "shocking".
About 88% of all UK adults, about 46 million people, used the net in the last three months, it said.
The ONS bulletin on net use in the first three months of 2016 revealed that 91% of people living in London went online regularly. By contrast, in Northern Ireland only 82% of people are regular users of online sites and services.
Pete Lee, from the ONS's surveys and economic division, said the statistics revealed some significant changes in usage and exposed those sectors of the population where net use remained low or patchy.
Net use among women aged 75 and over had grown by 169% since 2011, it found. However, it found that a significant number of older people who start using the net did not do so consistently. About 5% of those aged 75 or more had stopped using the net in early 2016.
"While we have seen a notable increase in internet usage across all groups in recent years, many older and disabled people are still not online," said Mr Lee.
Mark Atkinson, chief executive at disability charity Scope, said it was "shocking" that 25% of disabled people had not used the net as it created a significant digital divide.
"Digital access has the power to make life cost less," he said. "Many people go online to compare the best consumer deals and offers."
"Life costs more if you are disabled, from higher energy bills to specialist equipment. Scope research shows these costs add up to on average Â£550 per month," he added.
The US has denied that any of its unmanned spy planes have been shot down by Iran.
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Almost two-thirds of people aged over 75 have never gone online, suggest figures from the Office for National Statistics (ONS).