The Iranian Revolutionary Guards shot down a U.S. unmanned aircraft. on June 20 amid increasing tensions between Tehran and Washington over the failure of its nuclear agreement with world powers, said US and Iranian officials, while denying the circumstances of the incident.

Guards said shooting down unmanned aircraft over Iranian airspace, while two US officials said that the decline occurred in international airspace on the Strait of Hormuz. Different accounts cannot be directly reconciled.

Earlier, the US military alleged that Iran had fired missiles at another unmanned aircraft last week in response to an attack on two oil tankers near the Gulf of Oman. The US blamed Iran for the attack on the ships; Tehran denied involvement.

The attack comes against the backdrop of increasing tension between the US and Iran following President Donald Trump's decision to withdraw from the Tehran nuclear deal a year ago. The White House separately said it was aware of reports of missile attacks on Saudi Arabia in the midst of campaigns targeting the kingdom by Iranian-Iranian ally Houthi rebels.

Iran has recently doubled its low-enriched uranium production and threatened to increase its enrichment closer to the level of weapons, trying to pressure Europe for new requirements in the 2015 nuclear deal.

In the past few weeks, the US has accelerated an aircraft carrier to the Middle East and deployed additional troops to tens of thousands already in the region. From Yemen, the Houthis have launched bomb-loaded drones into neighboring Saudi Arabia.

All this raises concerns that miscalculations or increasing tensions can push the US and Iran into open conflict, about 40 years after the Tehran Islamic Revolution. Thursday's drone incident marked the first direct attack Iran claimed in the US in the midst of a crisis.

"We have no intention of fighting any country, but we are fully prepared to fight," said Revolutionary Guard commander General Hossein Salami in a televised speech.