Tehran and Washington have been locked in an escalating standoff since US President Donald Trump unilaterally withdrew from a multi-party 2015 nuclear deal with Iran and reimposed sanctions on the Islamic republic.

Tensions spiked last week when Iran shot down a US drone over sensitive Gulf waters following a series of tanker attacks that Washington blamed on Tehran, which has denied involvement.

Since then the arch-foes have been locked in a war of words, which escalated this week when Trump announced new sanctions against Iran's supreme leader Ayatollah Ali Khamenei and Foreign Minister Mohammad Javad Zarif.

Iran has threatened to abandon some of its commitments under the nuclear deal unless the remaining partners -- Britain, China, France, Germany and Russia -- help it circumvent US sanctions and especially sell its oil.

In May, the US Air Force deployed several nuclear-capable B-52 Stratofortress bombers to the Gulf in response to what the Defense Department described as a possible plan by Iran to attack American forces in the region, as well as an aircraft carrier task force. —AFP