TEHRAN: Tehran said on Monday that it has warned Washington against making another attempt to seize an Iranian tanker that has departed Gibraltar despite a US bid to have it detained.

"Iran has given necessary warnings to American officials through its official channels... not to make such a mistake because it would have grave consequences," Iran's foreign ministry spokesman Abbas Mousavi told a news conference.

Iran had been locked in a high-seas standoff with US ally Britain since Royal Marines seized the tanker off the coast of UK territory Gibraltar on July 4 on suspicion it was shipping oil to Syria in breach of EU sanctions.

Two weeks later, Iran's Islamic Revolutionary Guard Corps impounded a British-flagged tanker in strategic Gulf waters in what Britain called a tit-for-tat move.

Speaking to reporters on Monday, the Iranian foreign ministry spokesman dismissed the notion that there was a link between the two seizures.

"There is no connection whatsoever between these two vessels," said Mousavi.

"There have been two or three maritime violations made by that ship," he said, referring to the British-flagged 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."

The Iranian spokesman said the Gibraltar court order for the release of the tanker was a blow to US "unilateralism".

"The americans have not been very successful with their unilateral sanctions that have no legal basis.

"They should come to their senses that bullying and unilateralism cannot get anywhere in the world today."

Mousavi urged other countries not to accept sanctions the United States has imposed on Iran "because they're not legitimate and have no legal basis".

Tensions between Iran and its arch-foe the United States have been rising since President Donald Trump unilaterally withdrew the US from a landmark nuclear deal in May 2018 and began imposing sanctions against the Islamic republic as part of a "maximum pressure" campaign.

Iran has responded by suspending some of its commitments under the nuclear deal.

The situation has threatened to spiral out of control in the past few weeks with ships attacked, drones downed and oil tankers seized.

At the height of the crisis, Trump called off air strikes against Iran at the last minute in June after its forces shot down a US drone. —AFP