A US military surveillance drone has been shot down by Iranian forces while flying over the Strait of Hormuz, the BBC reports.
Iran's Islamic Revolution Guards Corps (IRGC) said the drone had violated Iranian airspace. But US military said it had been over international waters.
IRGC commander-in-chief Maj-Gen Hossein Salami said the downing of the drone sent a "clear message to America" that Iran's borders were its "red line".
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.
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.