Under the radar: Iranian oil tanker suspected of heading to Syria turns off tracking beacon

An Iranian oil tanker previously held by the UK has turned off its tracking beacon for more than 13 hours amid increasing suspicion the vessel is heading to Syria, despite assurances from Tehran that it would not.

The Adrian Darya 1, formerly known as the Grace 1, turned off its Automatic Identification system on Monday night, AFP reported.

The tracker - which previously showed the tanker off Lebanon on a course towards Tartous, Syria - has yet to be turned back on.

The former Grace 1 was detained off the coast of Gibraltar by the UK navy in early July on suspicion of heading towards Tartous to offload oil in contravention of European Union sanctions on the Syrian regime.

Gibraltar authorities ultimately released the Iranian oil tanker last month, despite repeated US requests to seize the tanker, claiming that it is controlled by the Iranian Revolutionary Guard, which it classifies as a terrorist organisation.

US Secretary of State Mike Pompeo has alleged the Adrian Darya will head to Tartous to offload its 2.1 million barrels of Iranian crude oil, worth some $130 million.

Iranian officials have not said where the ship will go, only that its cargo has been sold to an unnamed buyer.

The Islamic Republic has also said it could not name the actual destination due to "economic terrorism" by the US and its sanctions on Iran's oil sales.

Tensions between arch-enemies Iran and the US have soared ever since Washington stepped up its campaign of "maximum pressure" against Tehran and reimposed sanctions after leaving the landmark 2015 nuclear deal last year.

President Donald Trump in June called off at the last minute an airstrike on Iran over its downing of a US spy drone, amid a military and naval buildup by Washington and its ally the UK in the Gulf.

Iran has also captured several oil tankers off its coast in the past months, reportedly in retaliation for the seizure of the former Grace 1.