The British territory's Chief Minister Fabian Picardo said earlier that a supertanker suspected of carrying crude oil to Syria in violation of EU sanctions was detained in Gibraltar.

"After the illegal interception of an Iranian oil tanker in the strait of Gibraltar by the English naval force, that country's ambassador to Tehran was summoned to the foreign ministry," said the ministry's spokesman Abbas Moussavi.

Authorities in Gibraltar did not say where the oil came from but specialised shipping trade publication Lloyd's List said the Panamanian-flagged tanker was thought to be transporting crude from Iran.

The incident comes at a sensitive time as the European Union mulls how to respond to Iran announcing it will exceed the maximum uranium enrichment level it agreed in a landmark 2015 nuclear deal.

Washington abandoned the deal between Tehran and world powers last year and reimposed sanctions on the Islamic republic's crucial oil sales as well as other parts of the economy.

The Grace 1 tanker was halted by police and customs agencies in Gibraltar, a small rocky outcrop on Spain's southern tip.

They were aided by a detachment of British Royal Marines.