The Iranian oil tanker detained by British authorities for allegedly attempting to ferry oil to Syria has departed Gibraltar after the British overseas territory rejected a United States demand for it to continue to hold the vessel.

Although Gibraltar has not confirmed its departure, the monitoring website, Marine Traffic said the supertanker which had been detained since July 4 off the coast of Gibraltar set sail on Sunday evening.

Authorities in Gibraltar seized the Grace 1 on July 4 on suspicion that it was transporting oil to Syria in breach of European Union sanctions, triggering a sharp deterioration in relations between Tehran and London.

Last Thursday, the EU Supreme Court ordered that the tanker be released, with Iranian  officials saying that a new crew had arrived to pilot the vessel, now renamed the Adrian Darya and its 2.1 million barrels of oil.

However, on Friday, the US Justice Department filed a last-minute request to detain the ship, alleging that it was involved in supporting illicit shipments to Syria by Iran’s Islamic Revolutionary Guard Corps, which Washington had listed as a terrorist group.

Gibraltar’s government rejected that request, saying it could not seek a court order to detain the supertanker because US sanctions against Iran were not applicable in the European Union.

In its decision ordering the release of the tanker, Gibraltar’s government added that it had received written assurances from Iran that the ship would not be headed for countries subject to EU sanctions.

The July 4 seizure by Gibraltar authorities and by British Royal Marines came amid surging tensions in the Gulf after several alleged Iranian attacks on smaller tankers in the region. 

The US government, citing Tehran’s threat to American allies expanded its military presence in the region with a new aircraft carrier task force, missile batteries and strategic bombers.

Ties between Tehran and Washington have have further strained since last year after President Donald Trump withdrew from a landmark 2015 nuclear deal between major powers and Iran, reimposing crippling unilateral sanctions on the Asian country.

As of early Monday, the vessel had turned east, with Marine Traffic reporting its destination as Kalamata in Greece.