Britain has promised to release a detained Iranian oil tanker if Tehran guarantees the vessel will not go to Syria.

The tanker was seized last week by British Royal Marines off the coast of the British Mediterranean territory of Gibraltar on suspicion of violating sanctions against Syria.

Iran on Saturday reiterated its call for the ship’s release. It denies the tanker was taking oil to Syria in violation of EU sanctions.

British foreign minister Jeremy Hunt told his Iranian counterpart the Grace 1 oil tanker would be released but only if it does not travel to Syria.

The affair has stoked tension in the Gulf, with Britain saying on Thursday it had fended off Iranian ships that tried to block a British tanker in the area.

Mr Hunt said his call with Iran’s foreign minister Mohammad Javad Zarif was constructive, and Mr Zarif had told him Iran wanted to resolve the issue and was not seeking to escalate tensions.

“I reassured him our concern was destination not origin of the oil on Grace One & that UK would facilitate release if we received guarantees that it would not be going to Syria, following due process in (Gibraltar) courts,” Mr Hunt tweeted.

1/2 Just spoke to Iranian Foreign Minister Zarif. Constructive call. I reassured him our concern was destination not origin of the oil on Grace One &that UK would facilitate release if we received guarantees that it would not be going to Syria, following due process in Gib courts

— Jeremy Hunt (@Jeremy_Hunt) July 13, 2019

A statement on the Iranian foreign ministry website said Mr Zarif told Mr Hunt during the call that Britain should quickly release the tanker. Iran will continue its oil exports under any conditions, Mr Zarif also told Mr Hunt, according to the statement.

Tehran blames the US for arranging the tanker’s seizure. Washington has imposed sanctions against Iran with the aim of halting Iranian oil exports.

European countries do not have sanctions against Iran, but have had them in place against Iran’s ally Syria since 2011.

Gibraltar’s chief minister Fabian Picardo told the territory’s parliament on Friday the decision to detain the tanker, which he said was carrying 2.1 million barrels of oil, had not been taken at the request of any other country.

Gibraltan police said four crew members that had been arrested, including the vessel’s captain and chief officer, had been released on bail without charge but their investigation was ongoing.

-AAP