The British overseas territory rejected a US demand to seize the vessel, which has since departed Gibraltar and entered international waters.

"It's very unfortunate that that ship was released," Pompeo told Fox News when asked if the move was a mistake by the British.

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

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

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

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.