Iran has summoned the British ambassador in Tehran to complain about what it says is 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.
Spain’s acting foreign minister said the seizure of the ship – Grace 1 – was at the US’s request.
An Iranian foreign ministry spokesman said detaining its tanker was illegal.
BBC diplomatic correspondent Jonathan Marcus said a significant international row could be developing.
Gibraltar port and law enforcement agencies detained the super tanker and its cargo on Thursday morning with the help of the marines.
The BBC has been told 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.
A defence source described it as a “relatively benign operation” without major incident.
However, Iran’s foreign ministry spokesman Abbas Mousavi was later quoted on Iranian state TV as saying the UK’s ambassador in Tehran, Robert Macaire, had been summoned over the “illegal seizure” of the tanker.
Foreign Secretary Jeremy Hunt said the swift action by the authorities in Gibraltar and the Royal Marines would deny valuable resources to Syrian President Bashar al-Assad’s “murderous regime”.
Gibraltar said there was reason to believe the ship was carrying Iranian crude oil to the Baniyas Refinery in the Syrian Mediterranean port town of Tartous.
The refinery is a subsidiary of the General Corporation for Refining and Distribution of Petroleum Products, a section of the Syrian ministry of petroleum.
The EU says the facility therefore provides financial support to the Syrian government, which is subject to sanctions because of its repression of civilians since the start of the uprising against President Assad in 2011.
The refinery has been subject to EU sanctions since 2014.