The Iranian oil tanker at the center of a diplomatic dispute has departed from Gibraltar after the British overseas territory rejected a US demand to seize the vessel, which is hauling $130 million worth of light crude oil.

According to the monitoring website Marine Traffic, the supertanker -- which had been detained since July 4 off the coast of Gibraltar -- lifted anchor Sunday evening and started sailing south.

Gibraltar seized the Grace 1 on July 4 on suspicion it was transporting oil to Syria in breach of European Union sanctions.

Gibraltar authorities rejected an eleventh-hour attempt by the United States to reseize the oil tanker on Sunday, arguing that EU regulations are less strict than US sanctions on Iran.

Iranian officials say a new crew had arrived to pilot the vessel -- now renamed the Adrian Darya -- and its 2.1 million barrels of oil.

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

In its decision ordering the release of the tanker, Gibraltar's government said it had received written assurances from Iran that the ship would not be headed for countries "subject to European Union sanctions".

Iran denied it had made any promises about the ship's destination to secure the release.

"Iran has given no assurances over the Grace 1 not going to Syria to secure its release," a state media website quoted foreign ministry spokesman Abbas Mousavi as saying.

The US State Department has threatened to issue a visa ban on anyone working on the ship. US officials told reporters that the oil aboard the ship was worth some $130 million and that it was destined for a designated terror organization.

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

The US -- 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.

Iran subsequently detained the British-flagged tanker Stena Impero in what was seen as a tit-for-tat move.

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