An Iranian surface-to-air missile has shot down an American drone flying in international airspace over the Strait of Hormuz, a US official said.

Iran's Revolutionary Guard said earlier that it had brought down a US drone amid heightened tensions between Tehran and Washington over its collapsing nuclear deal.

A spokesman said the drone was flying in Iranian airspace, contradicting the American official.

The reported downing of the RQ-4 Global Hawk comes after the US military previously alleged Iran fired a missile at another drone last week which responded to the attack on two oil tankers near the Gulf of Oman.

The US blames Iran for the attack on the ships, which Tehran denies.

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

Iran recently has 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.

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

Mysterious attacks also have targeted oil tankers as Iranian-allied Houthi rebels launched bomb-laden drones into Saudi Arabia.

All this has raised fears that a miscalculation or further rise in tensions could push the US and Iran into an open conflict, some 40 years after Tehran's Islamic Revolution.

PA Media