The Adrian Darya 1, previously named Grace 1, headed east into the Mediterranean Sea on Monday hours after authorities in Gibraltar rejected a request from Washington to hold the ship.
Its destination was the Greek port of Kalamata, according to shipping data.
Abbas Mousavi, spokesman for Iran’s foreign ministry, told a news conference that any attempt at seizing the vessel, which has a cargo of 2.1 million barrels of crude oil, would have “grave consequences”.
“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 US officials through official channels, especially the Swiss embassy in Tehran, not to commit such a mistake because it would have grave consequences.”
Iran had denied the tanker was ever headed to Syria.
The British move triggered a sharp deterioration in relations between Iran and the United Kingdom, and Tehran subsequently detained a British-flagged tanker in what was seen as a tit-for-tat move.
Gibraltar’s Supreme Court ordered the release of the Iranian tanker last week after authorities in the British overseas territory said it had received written assurances from Iran that the ship would not be headed to countries subject to EU sanctions.
Tehran denied it had made any promises about the ship’s destination to secure the release.
In a last-ditch effort to stop the release, the US unsealed a warrant on Friday to seize the Adrian Darya 1 and its cargo, citing violations of US sanctions as well as money laundering and “terrorism” statutes.
The unsealed court documents argued that Iran’s Islamic Revolutionary Guard Corps (IRGC), which Washington has designated a “terrorist organisation”, are the ship’s true owners through a network of front companies.
But Gibraltar’s government rejected the request, saying it could not seek a court order to detain the supertanker because US sanctions against Iran were not applicable in the EU.
The row comes amid a growing confrontation between Iran and the US after President Donald Trump pulled Washington out of Tehran’s nuclear deal with world powers over a year ago. The pact’s remaining signatories – UK, France, Germany, Russia and China – oppose the US move and have pledged to protect Iran from sanctions reimposed by Washington.
Mousavi said the Gibraltar court order for the release of the oil tanker was a blow to US “unilateralism”.
He also denied any link between the seizure of the Iranian tanker off Gibraltar and the British-flagged tanker in the Gulf.
“There have been two or three maritime violations made by that ship,” he said, referring to the Stena Impero held off the Iranian port of Bandar Abbas.
“The court is looking into it. We hope the [investigation] is completed as soon as possible and the verdict is issued.”
Separately, a senior Iranian legislator said the crisis in Iran’s ties with Britain would not be over until the Adrian Darya 1 reached its destination.
“Until the Iranian oil tanker arrives at its destination the British must help end the crisis,” Heshmatollah Falahatpisheh, a member of parliament’s national security and foreign affairs committee, was quoted as saying by the semi-official ISNA news agency.
“This means that the crisis with Britain is not over. Britain has the primary responsibility for ending the oil tanker crisis,” Falahatpisheh said.
The final destination of the supertanker remains unclear, with authorities in Greece yet to confirm that it is expected to dock there.