A U.S. military drone has been shot down in international airspace over the Strait of Hormuz by an Iranian surface-to-air missile, a U.S. official said on Thursday, speaking on condition of anonymity.

The official said the drone was a U.S. Navy MQ-4C Triton. No further details were immediately available, including the time of the shoot-down.

Tehran confirmed the downing of the drone. Iran's elite Revolutionary Guards have shot down a U.S. "spy" drone in the southern province of Hormozgan, the Guards' news website Sepah News said.

"It was shot down when it entered Iran's airspace near the Kouhmobarak district in the south," the Guards' website added.

The country's state news agency IRNA carried the same report, identifying the drone as an RQ-4 Global Hawk.

Iran's borders "represent our red line", the head of the country's powerful Revolutionary Guards warned.

Tehran's response to the drone was "a clear message" from the "defenders of the borders" of Iran, the semi-official Tasnim news agency said, quoting Hossein Salami.

A senior Iranian security official said on Wednesday Iran would "strongly respond" to any violation of its airspace.

"Our airspace is our red line and Iran has always responded and will continue to respond strongly to any country that violates our airspace," Tasnim quoted the secretary of Iran's Supreme National Security Council as saying.

The RQ-4 Global Hawk unmanned aircraft system (UAS) can fly at high altitudes for more than 30 hours, gathering near-real-time, high-resolution imagery of large areas of land in all types of weather, according to its maker Northrop Grumman.

Tension between Iran and the United States has spiked since last year when President Donald Trump withdrew from a 2015 nuclear deal between Iran and major powers and reimposed sanctions on Iran.

Concern about a military confrontation has increased since attacks on two oil tankers in the Gulf of Oman last week and on four tankers off the United Arab Emirates on May 12, both near the Strait of Hormuz, a major conduit for global oil supplies.

The United States and its regional ally, Saudi Arabia, blamed Iran for the incidents. Iran has denied responsibility.

The U.S. 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.

Iran said last week that it was responsible for the security of the Strait of Hormuz, calling on American forces to leave the Gulf.

In protest at Trump's "maximum pressure", Iran said in May it would start enriching uranium at a higher level unless other European signatories to the nuclear deal protected its economy from the U.S. sanctions within 60 days.

(With input from Reuters and AFP)