GIBRALTAR -- An Iranian supertanker with $130 million worth of light crude oil that the U.S. suspects is tied to a sanctioned organization left Gibraltar and was heading east into the Mediterranean Sea on Monday, with its next destination reported to be Greece.

The Iran-flagged Adrian Darya 1, previously named Grace 1, set course for Kalamata, Greece, with an estimated arrival on Aug. 25, according to ship tracking service MarineTraffic. It wasn't immediately clear why the tanker would be heading there or whether the destination could change.

The vessel left Gibraltar late Sunday after having been detained for a month in the British overseas territory for allegedly attempting to breach European Union sanctions on Syria. Gibraltar authorities rejected attempts by the U.S. to seize the oil tanker again, arguing that EU regulations are less strict than U.S. sanctions on Iran.

Gibraltar said it had been assured by Iran that the tanker wouldn't unload its cargo in Syria.

Iranian government officials have yet to publicly acknowledge the ship's next destination, or where it will discharge its cargo of 2.1 million barrels of crude oil. Iran has denied it was ever headed for Syria.

Greece's Ministry of Shipping and Island Policy said it had received no notification that the supertanker is headed for Kalamata.

The tanker's release comes amid a growing confrontation between Iran and the West after President Donald Trump pulled Washington out of Tehran's nuclear deal with world powers over a year ago. The decision re-imposed sanctions on Iran, stopping billions of dollars in business deals, largely halting the sale of Iran's crude oil internationally and sharply depreciating Iran's currency, the rial.

The U.S. Department of State reiterated its position Monday that the Adrian Darya 1 was "transporting illicit oil to fuel the Iranian regime's and Syrian regime's campaigns of terror and oppression," and it said that companies and mariners who assist it could be considered to be providing material support to a U.S.-designated Foreign Terrorist Organization.

"We have conveyed our strong position to the Greek government on the matter, as well as all ports in the Mediterranean that should be forewarned about facilitating this vessel," it said.

Iranian Foreign Minister Mohammad Javad Zarif said that his country was being discreet about the tanker's designation due to sanctions by the U.S., which he said "illegally tries to bully others from purchasing our oil."

The threat of punitive measures by the U.S. for buying Iranian oil has discouraged many countries from purchasing it, though the oil itself is not subject to any U.N. or international sanctions.

Zarif said the tanker's seizure in early July by British Royal Marines "was not based on any law," speaking in English to reporters in Helsinki, Finland.

Shortly after the ship was detained, Iran seized the British-flagged oil tanker Stena Impero, which remains held by the Islamic Republic. Analysts had said the Iranian ship's release by Gibraltar might mean that the Stena Impero could go free.

But Iranian officials have denied there had been a tit-for-tat seizure, alleging that the Stena Impero was detained after it violated international maritime law while transiting the Strait of Hormuz.

"There is no specific relation between these two ships," Foreign Ministry spokesman Abbas Mousavi said, referring to the Stena Impero and the Adrian Darya 1. He said the release of the Stena Impero depended on a court ruling about "two to three nautical violations that are being investigated."

In Gibraltar, a lawyer representing the former captain and two crew members of the supertanker said they had decided they no longer wanted to work on the ship, following the incident.

"They had had enough," lawyer Richard Wilkinson told The Associated Press on Monday. "(The captain) didn't feel happy about going back under these circumstances. He felt that he'd lost control. And being the person responsible for control, he didn't like that."

In a last-ditch effort to stop the Adrian Darya 1's release, the U.S. unsealed a warrant Friday to seize the supertanker and its cargo, citing violations of U.S. sanctions as well as money laundering and terrorism statutes.

U.S. 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 unsealed court documents argued that Iran's Islamic Revolutionary Guard Corps are the ship's true owners through a network of front companies.

Authorities in Gibraltar said Sunday that, unlike in the U.S., Iran's Revolutionary Guard is not designated a terrorist organization under EU, U.K. or Gibraltar law.

The Iranian spokesman warned Monday against any order by the U.S. Justice Department to have the renamed ship seized again.

"If such an action is taken or even if it is stated verbally and not done, it is considered a threat against the maritime security in international waters," said Mousavi. "The Islamic Republic of Iran has given necessary warnings to the U.S. officials through official channels, especially the Swiss Embassy in Tehran, not to commit such a mistake which (could) bring them severe consequences."

The Iranian ship was detained while sailing under a Panamanian flag with the name Grace 1. It changed the name on Sunday and hoisted an Iranian flag.

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Giles reported from Madrid. AP reporters Jan M. Olsen in Copenhagen, Denmark, and Nicholas Paphitis in Athens contributed to this report.