Iran's Revolutionary Guard shot down a US surveillance drone on Thursday amid heightened tensions between Tehran and Washington, with both sides disputing the circumstances of the incident.

Iran's paramilitary Revolutionary Guard, which answers only to Supreme Leader Ayatollah Ali Khamenei, said it shot down the drone at 4:05 am on Thursday when it entered Iranian airspace near the Kouhmobarak district in the southern Hormozgan province. Kouhmobarak is about 1,200 kilometres southeast of Tehran.

The drone took off from the southern Persian Gulf and collected data from Iranian territory, including the southern port of Chahbahar near the border with Pakistan, the Guard said.

The US military has not commented on the mission of the remotely piloted aircraft that can fly higher than 10 miles in altitude and stay in the air for over 24 hours at a time.

Iran used its air defence system known as Third of Khordad ─ a truck-based missile system that can fire up to 30km into the sky ─ to shoot down the drone, the semi-official Fars news agency reported.

Typically, militaries worldwide call out to errant aircraft entering their airspace before firing. It's unclear whether Iran gave any warning to the US drone before opening fire.

Iran's state-run IRNA news agency, citing the Guard, identified the drone as an RQ-4 Global Hawk.

"We do not have any intention for war with any country, but we are fully ready for war," Revolutionary Guard commander Gen. Hossein Salami said in a televised address.

Iran's foreign ministry spokesperson Abbas Mousavi was quoted by the semi-official Tasnim news agency as saying that Iran cannot condone the "illegal trespassing and invading of the country's skies by any kind of foreign flying object."

Mousavi expressed Iran's "strong objection" and added that the "invaders will bear full responsibility".

The US military's Central Command (Centcom) confirmed that Iranian forces shot down a US Navy surveillance drone, an RQ-4 Global Hawk, but insisted that the downing took place in international airspace, and described it as an "unprovoked attack".

A BAMS-D drone was downed by an Iranian surface-to-air missile system while flying in international airspace over the Strait of Hormuz, US Centcom spokesperson Navy Captain Bill Urban said in a statement. He said it happened at 23:35 GMT on Wednesday.

"Iranian reports that the aircraft was over Iran are false," he added. "This was an unprovoked attack on a US surveillance asset in international airspace."

Previously, the US military alleged that Iran had fired a missile at another drone last week that was responding to the attack on two oil tankers near the Gulf of Oman. The US blames Iran for the attack on the ships, while Tehran denies it was involved.

The attacks come against the backdrop of heightened tensions between the US and Iran following President Donald Trump's decision to withdraw from Tehran's nuclear deal a year ago.

The Trump administration has imposed sweeping sanctions on Iran, including trying to stop all exports of its oil, although it has issued a waiver for energy-starved Iraq to keep buying power from its neighbour.

Iranian lawmakers in April approved a bill labelling US forces in the Middle East as 'terrorist' after the US blacklisted the Revolutionary Guard as a terrorist organisation.

In recent weeks, the US has sped an aircraft carrier to the Mideast and deployed additional troops to the tens of thousands already in the region.

Iran recently quadrupled its production of low-enriched uranium and threatened to boost its enrichment closer to weapons-grade levels, trying to pressure Europe for new terms to the 2015 deal.

All this has raised fears that a miscalculation or further rise in tensions could push the US and Iran into an open conflict.

Reported missile strike in Saudi Arabia

The White House separately said it was aware of reports of a missile strike on Saudi Arabia amid a campaign targeting the kingdom by Yemen's Houthi rebels.

A Saudi-led coalition has been battling the Houthis since March 2015 in Yemen, the Arab world's poorest nation now, pushed to the brink of famine by the conflict.

In recent weeks, the Houthis have launched a new campaign sending missiles and bomb-laden drones into Saudi Arabia.

Spokeswoman Sarah Sanders said Trump had been "briefed on the reports of a missile strike in the Kingdom of Saudi Arabia."

"We are closely monitoring the situation and continuing to consult with our partners and allies," Sanders said.

The Houthi's Al-Masirah satellite news channel claimed the rebels targeted a power plant in Jizan, near the kingdom's border with Yemen, with a cruise missile.

However, Saudi Arabia said the rocket targeted a desalination plant, but that no one was wounded and the rocket caused no damage.

The state-run Saudi Press Agency reported the attack on Thursday, quoting military spokesman Col. Turki al-Maliki. The attack took place late Wednesday night. Saudi state media and officials did not immediately report the strike last night.