IRIB, a state broadcaster, aired a short video purporting to show the interception of the drone by an Iranian anti-missile battery.

The six second sequence showed the luminous nighttime trail of what appeared to be a missile before an explosion at very low altitude.

But the video was not broadcast by state TV's Farsi channel IRINN, which largely ignored the story.

Relevant units acted "in response to a violation of our airspace by a drone (that) infiltrated", IRIBNEWS reported, citing Brigadier General Alireza Sabahi Fard, commander in chief of aerial defence.

"The drone was shot down before it could reach sensitive sites thanks to the great vigilance of our unified aerial defence system," he said.

Earlier, the semi-official ISNA news agency reported that aerial defence had "destroyed an unidentified drone" which flew through the skies of Bandar-e Mahshahr in Khuzestan province.

Located near Iraq, Bandar-e Mahshahr and the neighbouring port of Bandar-e Emam-Khomeiny is a key zone for petrochemical activity.

The army downed the unmanned aircraft over Khuzestan province before dawn using a Mersad surface-to-air missile, reported the Tasnim news agency, which is close to Iran's ultra-conservatives and generally well informed on defence matters.

- Debris examined -

Citing provincial governor Gholamreza Shariati, it reported that the armed forces had located the wreckage of the drone in a marshy area and opened an inquiry into the incident.

Tasnim cited "unofficial" sources saying the missile that destroyed the drone had been fired "as part of a nighttime combat exercise aiming to test aerial defence systems". The agency later deleted those details from its site.

According to state news agency IRNA, Shariati said the "unknown drone... (belonged) without doubt to a foreign country."

The accidental crash of an Iranian drone in the same province on October 23 prompted newspaper reports that an unidentified foreign aircraft had been shot down.

The conservative Fars news agency later revealed that the drone that crashed in October was Iranian and had been forced to make an emergency landing.

The incidents come amid soaring tensions between Tehran and Washington since President Donald Trump unilaterally abandoned a landmark 2015 nuclear deal last year and reimposed crippling sanctions.

Trump in June authorised a military strike after Iran shot down a US drone, only to call off the retaliation at the last moment.

Washington insisted the aircraft was in international airspace, but Iran said it was over its territorial waters.

The crisis deepened with the September 14 attacks on energy giant Aramco's Abqaiq processing plant and Khurais oilfield in Saudi Arabia, which halved the kingdom's crude output.

Yemen's Iran-backed Huthi rebels claimed responsibility, but the US said the attacks involved cruise missiles from Iran and amounted to "an act of war". Iran denied any involvement it was behind the Khurais and Abqaiq attacks.

Washington has responded with a military build-up in the Gulf and has launched an operation with its allies to protect navigation in Gulf waters.

Since May, the United States has increased its 70,000-strong presence in the Middle East by 14,000 personnel, according to Pentagon figures, most of them deployed to the Gulf region.