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.
It was the second time in a month tankers have 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.

Tensions flared with the U.S. pullout last year from world powers’ 2015 nuclear accord with Iran and have worsened as Washington imposed fresh sanctions to throttle Tehran’s vital oil trade and Iran retaliated earlier this week with a threat to breach limits on its nuclear activities imposed by the deal.

Upping the ante, Washington said on Monday it would deploy about 1,000 more troops, along with Patriot missiles and manned and unmanned surveillance aircraft, to the Middle East on top of a 1,500-troop increase announced after the May tanker attacks.

Sepah News, the website of Iran’s elite Revolutionary Guards Corps (IRGC), said the “spy” drone was brought down over the southern Iranian province of Hormozgan, which is on the Gulf.