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 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. 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 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.

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 shot down a US “spy” drone in the early hours of Thursday after the unmanned aircraft violated Iranian airspace near Kuhmobarak in the southern province of Hormozgan.

The drone was identified by the IRGC as a RQ-4 Global Hawk, but the US military official told Reuters news agency the drone was a US Navy MQ-4C Triton, a maritime patrol and reconnaissance aircraft based on the RQ-4B Global Hawk.

Later, in a speech carried live on Iranian state TV, Gen Salami warned the US that it needed to respect Iran’s territorial integrity and national security.

“The downing of the American drone was a clear message to America, and the message is this: those who defend the borders of the Islamic nation of Iran will react in a total and decisive way to any intrusions by foreign elements on our land. Our borders are our red line.”

He added: “Iran is not seeking war with any country, but we are fully prepared to defend Iran.”

Iran’s foreign ministry warned of the consequences of “provocative measures”.