Iran has threatened to seize a British tanker after Royal Marines held one of their vessels suspected of taking oil to Syria.

Mohsen Rezai, a senior commander in Iran's elite Revolutionary Guards, said on Twitter: "If Britain does not release the Iranian oil tanker, it is the (Iranian) authorities' duty to seize a British oil tanker."

It comes as the crew of the seized ship, Grace 1, are being interviewed by officials.

British Royal Marines seized the ship off the coast of Gibraltar on Thursday for trying to take oil to Syria in violation of European Union sanctions.

A spokesman for the Gibraltar government said the 28-strong crew of the Iranian oil tanker are being interviewed as witnesses, not criminals.


Image: Royal Marines fast-roping onto the oil tanker in the early hours of Thursday

The spokesman said the crew, who have remained on board the super tanker, were mainly Indians with some Pakistanis and Ukrainians.

Police and customs officials remained on board the vessel to carry out their investigation but the Royal Marines have been stood down.

Gibraltar's Supreme Court has granted a 14-day extension for authorities to detain the tanker.

European Union sanctions against the government of Syria took effect in May 2011, shortly after the start of President Bashar al Assad's bloody crackdown on pro-democracy demonstrators that has led to the country's protracted civil war.

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Iran, a close ally of Assad, is also under a US sanctions regime meant to bar all international sales of Iranian oil.

It was imposed after US President Donald Trump withdrew Washington from a nuclear deal with Tehran last year.