The United States has sanctioned an Iranian oil tanker, which was earlier released by Gibraltar, after a five-week standoff over whether it was carrying Iranian oil for Syria in violation of European Union's sanctions.

The US Department of Treasury said the vessel, previously known as the Grace 1, is "blocked property" under the anti-terror order and "anyone providing support to the Adrian Darya 1 risks being sanctioned," according to an AFP report.

Tensions between Tehran and Washington have risen since US President Donald Trump withdrew from a landmark 2015 nuclear deal between major powers and Iran last year and imposed crippling unilateral sanctions.

Two weeks ago, the US threatened a visa ban on the crew of the tanker.

Washington says the tanker is carrying crude ultimately benefiting Iran's Islamic Revolutionary Guard Corps- Quds Force, which it has listed as a terrorist organisation.

Illicit Trade

"Vessels like the Adrian Darya 1 enable the IRGC-QF to ship and transfer large volumes of oil, which they attempt to mask and sell illicitly to fund the regime's malign activities and propagate terrorism," said Sigal Mandelker, Under Secretary for Terrorism and Financial Intelligence.

Adrian Darya's captain, Akhilesh Kumar, is also sanctioned under the order, which generally bans dealings with the blocked property by US persons. "In addition, persons that engage in certain transactions with the individuals and entities designated today may themselves be exposed to sanctions or subject to an enforcement action," the treasury department said.

Faced with US warnings, the tanker has been bouncing around the Mediterranean with its cargo of 2.1 million barrels of crude oil.

Washington earlier said that if the tanker's oil was sold, the revenue would be used by elite units of Iran's Revolutionary Guards. The US wants to deny them the resources to continue their activities.

Soon after Gibraltar lifted the detention order, a US federal court ordered the seizure of the vessel on different grounds, but that petition was rejected by Gibraltar.

The European Union bans oil sales to Syria, while the United States has sanctions on Iranian oil sales.