The US State Department has confirmed that one of its officials offered millions of dollars to the captain of an Iranian oil tanker which is at the centre of a diplomatic row.

America wanted the captain to sail the vessel to a country that would impound and seize the ship on behalf of the United States.

The US wants to seize the Adrian Darya 1, formerly known as the Grace 1, as it suspects it of being on its way to ship oil £107 million ($130 million) to Syria.

Authorities in Gibraltar seized the tanker in July and released it in August when officials there said they received assurances its oil would not go to Syria.

However, Iranian officials have not identified who bought the Adrian Darya's cargo, only that it has been sold.

Authorities in Gibraltar rejected the United States’ latest request not to release the supertanker Credit: AP

The US tried every means to prevent the Adrian Darya 1 tanker from leaving Gibraltar in August, but the authorities there rejected an eleventh-hour attempt by the United States to reseize the oil tanker, arguing that EU regulations are less strict than US sanctions on Iran.

After leaving Gibraltar, the Adrian Darya 1 headed across the Mediterranean and was last seen off the coast of Syria, before turning off its tracking beacon and disappearing from view.

US intelligence services believe the vessel is waiting for a ship-to-ship transfer of the oil, which will ultimately see its cargo end up in Syria.

However, the efforts of the US to seize the vessel have not stopped and Washington has now admitted that it contacted Akhilesh Kumar, the captain of the ship, and offering him several million dollars if he pulled into a harbour where the boat could be seized.

Mr Kumar refused, and was sanctioned by Washington.

Iranian Foreign Minister Javad Zarif responded by saying the US had "failed at piracy" and was now resorting "to outright blackmail".

"Deliver us [the US] Iran's oil and receive several million dollars or be sanctioned yourself," he added.

After it came to light, US Special Representative for Iran, Brian Hook, the man who first sent the email did not withdraw the tactic, but instead expanded on it.

"We are announcing a reward of up to $15 million for any person who helps us disrupt the financial operations of Iran's Revolutionary Guard and Quds Force," Mr Hook told a briefing.

The US considers Iran's Revolutionary Guard and its Quds Force - a unit within it that carries out unconventional warfare and intelligence activities, and is responsible for extraterritorial operation - terrorist organisations.

This policy of "maximum pressure" on Iran was also being applied in London on Thursday, with the visit of Israeli Prime Minister Benjamin Netanyahu.

Mr Netanyahu wants to dissuade European countries, like the UK, from easing sanctions on arch-rival Iran and trying to save the nuclear deal with Tehran.

Crew members of the Grace 1 remove the name of the ship. Credit: AP

The 2015 Iran Nuclear Deal has been in trouble for more than a year after US President Donald Trump pulled Washington out of the accord.

Under the landmark 2015 agreement, Iran agreed to limit its enrichment of uranium in exchange for the lifting of economic sanctions.

When America pulled out of the deal, it imposed sanctions on Iran, one of which is blocking it from selling its crude oil abroad.

The sanctions the US has reimposed on Iran are having a crippling effect on its economy.

Meanwhile, tensions have spiked across the Persian Gulf over mysterious tanker explosions, the shooting down of a US military surveillance drone by Iran, and America deploying more troops and warplanes to the region.

Iran, too, is upping the ante and taking another step towards abandoning the 2015 deal by saying it would lift all limits on its development of centrifuges which enrich uranium.

The International Atomic Energy Agency confirmed last week that Iran's stockpile of low-enriched uranium still exceeds the amount allowed by the deal.

The UN agency also said Iran continues to enrich uranium up to 4.5%, above the 3.67% allowed under the deal but still far below weapons-grade levels of 90%.

On Friday, Iran warned it will take additional steps away from the accord if it does not get help from Europe to sell its oil abroad, calling it their "third step" away from the deal.