An Iranian oil tanker detained off Gibraltar last month left the British territory and was headed through the Mediterranean towards Greece on Monday.

Tehran warned any attempt by the US to seize the Adrian Darya 1 again would be met with "heavy consequences".

The vessel, previously known as the Grace 1, set off at around 11pm local time on Sunday, Refinitiv shipping data showed.

It slowly steered southeast toward a narrow stretch of international waters separating Morocco and the southern tip of the Iberian Peninsula.

Its destination was not immediately clear, but tracking data suggested it was travelling in the direction of Kalamata in Greece.


Image: The tanker has been renamed the Adrian Darya 1

Asked whether the US could renew its seizure request, Iran's foreign ministry spokesman Abbas Mousavi said: "Such an action, and even the talk of it... would endanger shipping safety in open seas.

"Iran has issued the necessary warnings through official channels, especially the Swiss embassy, to American officials not to commit such an error because it would have heavy consequences."

Switzerland represents American interests in Iran, which has no diplomatic relations with the US.

The vessel, which is hauling $130m (£106m) worth of light crude oil, had been in Gibraltar since 4 July amid speculation that it planned to transport its cargo to Syria, in breach of European Union sanctions.

The detention of the vessel prompted Iran to retaliate by seizing the British-flagged oil tanker Stena Impero, heightening already increased tensions in the region.

Reports suggested the release of the Iranian ship might mean the Stena Impero could go free.

But Mr Mousavi denied there was any link between the two vessels: "We are glad that our stance about the unlawful and illegal seizure of our tanker has been proven.

"Regarding the release of the law-breaking British tanker [Stena Impero], we have to wait for the court's ruling.

"This tanker has committed two to three nautical violations that are being investigated. We hope that these investigations will finish as soon as possible and a verdict will be issued and if the verdict orders its release, it can continue to sail its path."

Gibraltar defies US and releases seized Iranian tanker

Earlier, Iran threatened to dispatch its naval fleet to collect its vessel.

"The era of hit and run is over," said Iran's navy commander Rear Admiral Hossein Khanzadi, according to the Mehr news agency.

"If top authorities ask the navy, we are ready to escort our tanker Adrian," he added.

Authorities in Gibraltar said they had rejected a renewed request from the US to keep the tanker under detention.

It comes after the US had issued a warrant to seize the Adrian Darya 1 on Saturday.

The US said it could seize the vessel, its oil cargo and almost $1m (£823,000) on the grounds of terrorism and violations of the International Emergency Economic Powers Act.

It claimed the tanker had links to the Islamic Revolutionary Guard Corps (IRGC), which it deems a terrorist organisation.

But in a statement earlier on Sunday, Gibraltar's government said US sanctions had no equivalent in the UK or the rest of the EU.

"The EU sanctions regime against Iran... is much narrower than that applicable in the US," it said.

"The Gibraltar Central Authority is unable seek an Order of the Supreme Court of Gibraltar to provide the restraining assistance required by the United States of America."

Video and photographs showed the tanker flying the red, green and white flag of Iran and bearing its new name, painted in white, on the hull.

The impounding of the vessel sparked a diplomatic row that escalated when Tehran seized the Stena Impero.

It has fed into wider hostilities since the US last year pulled out of an international agreement to curb Iran's nuclear programme, and reimposed economic sanctions.

Iran has denounced US efforts to set up an international maritime security coalition in the Gulf and insisted regional countries could protect the strategic waterway and work towards signing a non-aggression pact.

The Adrian Darya 1 had originally flown Panama's flag but the country's maritime authority said last month that the vessel had been de-listed after an alert that indicated the ship had participated in or was linked to terrorism financing.