Iran's elite Revolutionary Guard force said Thursday it shot down a US "spy drone" over its territory, Iranian state television reported.
An anonymous US official later told news agencies an American naval drone was downed over international airspace.
"The US-made Global Hawk surveillance drone was brought down" in the country's southern coastal province of Hormozgan, the Revolutionary Guard was quoted as saying by the English-language Press TV.
"It was shot down when it entered Iran's airspace near the Kouhmobarak district in the south," the force's website said.
State television did not provide images of the aircraft.
The US military initially denied the report.
"There was no drone over Iranian territory," Navy Captain Bill Urban, a US Central Command spokesman, told The Associated Press. He declined further comment.
However an American official, speaking on condition of anonymity, later said a US Navy MQ-4C Triton drone was brought down by an Iranian surface-to-air missile in international airspace over the Strait of Hormuz.
No further details were immediately available, including the time of the shootdown.
A senior Iranian security official said on Wednesday that Iran would "strongly respond" to any violation of its territory.
"Our airspace is our red line and Iran has always responded and will continue to respond strongly to any country that violates our airspace," the semi-official Tasnim news agency quoted the secretary of Iran's Supreme National Security council as saying.
Thursday's shootdown comes amid heightened tensions between Iran and the United States after President Donald Trump last year pulled out of a historic 2015 nuclear deal between Tehran and major powers and reimposed sanctions on the country.
The US military has sent forces, including aircraft carriers, B-52 bombers and troops to the Middle East. However, Trump said he does not seek war with Iran.
Fears of conflict have risen after two oil tankers came under attack a week ago near the Strait of Hormuz - a major oil shipping route where one-fifth of the world's oil passes from the Middle East to world markets.
The US and its regional allies - Saudi Arabia and the United Arab Emirates (UAE) - have accused Iran of being behind the series of blasts in the Gulf.
Tehran has denied involvement and instead suggested Washington could be responsible, using it to justify force against Iran.
On Wednesday, the US Navy said recovered fragments from one of two tanker ships bore a "striking resemblance" to mines seen during Iranian military parades.
Iran has repeatedly denied any responsibility in the June explosions, as well as similar blasts on May 12 off the coast of the UAE that targeted four oil-carrying vessels.
Meanwhile, US Special Representative for Iran Brian Hook headed to the Middle East for meetings in Saudi Arabia, UAE, Oman, Kuwait and Bahrain to discuss "Iran's regional aggression", the State Department said.
"He will also share additional US intelligence on the range of active threats Iran currently poses to the region," it said in a statement.
In protest at Trump's "maximum pressure" sanctions strategy, Iran announced in May it would start enriching uranium at a higher level unless European signatories to the nuclear deal protected its oil and banking sectors within 60 days.
The United States has vowed that Iran will never possess nuclear weapons.