Release of Iranian oil tanker unfortunate: Pompeo

US Secretary of State Mike Pompeo has called the release of an Iranian oil tanker "unfortunate", as the ship which was detained by Gibraltar in July on suspicion of transporting oil to Syria, was on its way to Greece after leaving port in the British territory.

"It's very unfortunate that that ship was released," Xinhua news agency quoted Pompeo as telling Fox News on Monday.

Data from ship-tracking service MarineTraffic showed that the oil tanker, which has changed its name from Grace 1 to Adrian Darya-1, set sail shortly before midnight on Sunday towards international waters and was heading for the Greek port of Kalamata.

Gibraltar authorities freed the Adrian Darya-1 on August 15 after receiving assurances from Iran that it would not discharge its cargo in Syria.

The next day, the US Justice Department filed a request to detain the ship on the grounds that it had links to Iran's Islamic Revolutionary Guard Corps (IRGC), which it has designated a terrorist group.

But Gibraltar in a statement on Sunday, said it could not comply with the request because the IRGC was not viewed as a terrorist organisation by the EU, which the British territory is currently part of, the BBC reported.

In his interview on Monday, Pompeo claimed that if Iran was successful in making profits by selling the oil from the tanker, Tehran's elite forces will "have more money, more wealth, more resources to continue their terror campaign, to continue their assassination campaign in Europe".

A US State Department official told the media on Monday that Washington has expressed its "strong position" to Greek authorities and all ports in the Mediterranean.

The official said assisting the Iranian tanker could be viewed as material support to a US-designated foreign terrorist organization, according to US media reports.

The Iranian Foreign Ministry on Monday warned against any attempt by the US to seize the Iranian oil tanker.

"Any attempt to do this (seize the Iranian vessel) would be a threat to the free maritime shipping," Ministry Spokesman Abbas Mousavi said.

The tanker, with its cargo of 2.1 million barrels of oil, and crew of 29 from India, Russia, Latvia and the Philippines, was seized with the help of British marines on July 4, after the government of Gibraltar, a UK territory, suggested it was heading for Syria in breach of European Union (EU) sanctions.

The seizure of the Adrian Darya-1 sparked a diplomatic crisis between the UK and Iran, which has escalated over recent weeks and saw Tehran seize a British-flagged and Swedish-owned oil tanker, Stena Impero, in the Gulf.

The Stena Impero, which was seized by the IRGC on July 19, remains in Iranian hands.