Iran has summoned the British ambassador here to complain about what it says was the illegal seizure of an Iranian oil tanker.
British Royal Marines helped the authorities in Gibraltar seize the ship because of evidence it was heading to Syria in breach of EU sanctions, the BBC reported on Thursday.
Spain's acting foreign minister said the seizure of the ship - Grace 1 - was at the US's request. An Iranian foreign ministry spokesman called the actions a "form of piracy". The UK Foreign Office dismissed that as "nonsense".
Gibraltar port and law enforcement agencies detained the super tanker and its cargo on Thursday morning with the help of the marines. According to BBC, a team of about 30 marines, from 42 Commando, were flown from the UK to Gibraltar to help, at the request of the Gibraltar government.
The first marines to board the Panama-flagged ship descended by rope from a helicopter, as others approached in speed boats. No shots were fired.
A defence source described it as a "relatively benign operation" without major incident.
However, Iran's foreign ministry spokesman Abbas Mousavi was later quoted as saying the UK's ambassador in Tehran, Robert Macaire, had been summoned over the "illegal seizure" of the tanker.
In a brief interview for Iranian TV's Channel Two, Mousavi said the seizure was "a form of piracy" and did not have any legal and international basis. He called for the tanker to be immediately released to continue its journey.
He added that "the move indicated that the UK follows the hostile policies of the US, which is unacceptable for the Iranian nation and government".
Spain's Acting Foreign Minister Josep Borrell said Spain was studying the circumstances of the action but said it followed "a demand from the US to the UK". Spain disputes British ownership of Gibraltar.
White House national security advisor John Bolton said the seizure was "excellent news", saying the tanker laden with oil bound for Syria was violating EU sanctions. He added that the US and its allies would continue to prevent regimes in Tehran and Damascus from "profiting off this illicit trade".