The US military identified the drone as a US Navy RQ-4A Global Hawk (file photo)
A US military surveillance drone has been shot down by Iranian forces while flying over the Strait of Hormuz.
Iran's Islamic Revolution Guards Corps (IRGC) said the aircraft had violated Iranian airspace, and that the incident sent a "clear message to America".
But the US military insisted the drone had been over international waters at the time, and condemned what it called an "unprovoked attack" by the IRGC.
It comes at a time of escalating tension between the US and Iran.
On Monday, the US defence department said it was deploying 1,000 extra troops to the region in response to "hostile behaviour" by Iranian forces. It has already sent an aircraft carrier strike group and B-52 bombers.
The US has also accused Iran of attacking two oil tankers with mines last Thursday just outside the Strait of Hormuz, in the Gulf of Oman. Iran rejects the allegation.
It was the second time in a month tankers had been attacked close in the region, through which a fifth of the world's oil passes each day.
Tensions were further fuelled on Monday when Iran announced its stockpile of low-enriched uranium would next week exceed limits it agreed with world powers under a landmark nuclear deal in 2015.
Iran stepped up its production in response to tightening economic sanctions from the US, which unilaterally withdrew from the deal last year. What happened on Thursday? The IRGC said its air force had shot down a US "spy" drone in the early hours after the unmanned aircraft violated Iranian airspace near Kuhmobarak in the southern province of Hormozgan.
In a speech carried live on Iranian state television, IRGC commander-in-chief Maj-Gen Hossein Salami warned the US to respect Iran's territorial integrity.
The US military's Central Command later confirmed that a US Navy Broad Area Maritime Surveillance (BAMS-D) aircraft was shot down by an Iranian surface-to-air missile system while operating in what it said was international airspace over the Strait of Hormuz at approximately 23:35 GMT on Wednesday (04:05 Iran time on Thursday).
"Iranian reports that the aircraft was over Iran are false," spokesman Navy Capt Bill Urban said. "This was an unprovoked attack on a US surveillance asset in international airspace."
The BAMS-D is a RQ-4A Global Hawk High-Altitude, Long, Endurance (HALE) drone that can carry out surveillance and reconnaissance missions over vast ocean and coastal regions, according to the US military. Captured RQ-170
This is not the first time that Iran is standing up to US.
In December 2011, US stealth drone on a spying mission 140 miles inside Iran was hacked and made it to land inside Iran.
The RQ-170 "Sentinel" drone was then placed under the watch of the Iranian Revolutionary Guards (IRGC) who a few years later, in May 2014, announced that they had reverse-engineered and made a clone of the drone.