The Iranian-flagged oil tanker Grace 1 – now called the Adrian Darya-1 – has deactivated its tracking beacon off the coast of Syria, leading to speculation that its cargo of crude oil will be delivered to President Bashar Al Assad's regime in defiance of EU sanctions.
Authorities in Gibraltar released the tanker in August after receiving "written assurances" from Tehran that the cargo would not be delivered to Syria.
Britain's Foreign Office said at the time that the UK would not "stand by and allow Iran – or anyone – to bypass vital EU sanctions on a regime that has deployed chemical weapons against its own people".
The vessel, carrying 2.1 million barrels of Iranian crude worth about $130 million (Dh447.4m), switched off its Automatic Identification System on Monday afternoon, according to the ship-tracking website MarineTraffic.com.
At its last indicated position, the ship was heading north in international waters, 83 kilometres off the coast of Lebanon and Syria.
US Secretary of State Mike Pompeo said last week that Washington had "reliable information" that the Grace 1 was heading to the Syrian port of Tartus, the deepest port in the eastern Mediterranean.
American officials were surprised when the Gibraltar court released the vessel and Mr Pompeo said it was a "big mistake" for the UK to trust Iranian Foreign Minister Javad Zarif
Grace 1's actions follow a pattern of other Iranian ships turning off their trackers near Cyprus, said Ranjith Raja, a lead analyst at the data firm Refinitiv.
Historically, Syria has taken about 1 million barrels of crude oil from Iran every month and Mr Raja said it was likely the ship would be offloading its cargo there.
The oil could be transferred to smaller vessels, allowing it to be taken to port, he said.
"The Iranian oil going to Syria is not something new,” Mr Raja said. "This is a known fact."
The oil shipment website Tanker Trackers also believes the Grace 1 to be off Syria.
In a tweet on Tuesday, after the ship’s beacon was disabled, Tanker Trackers said: "It is now safe to assume she is in Syria’s territorial waters."
Iranian officials have not yet identified who bought the ship’s cargo, only that it has been sold.
The US, which sought to seize the tanker, alleged in a federal court that the ship is owned by Iran’s Islamic Revolutionary Guard Corps, a paramilitary organisation answerable only to supreme leader Ayatollah Ali Khamenei.
Washington recently listed the paramilitary group a terrorist organisation, giving it greater power to pursue its assets.
Officials in the US issued repeated warnings to countries not to aid the Grace 1, which previously indicated it would be heading to Greece and Turkey before turning off its tracker on Monday.
Authorities in Gibraltar said the ship was bound for a refinery in Baniyas, Syria, when it was seized in early July.