Iran demanded on Friday that Britain immediately release an oil tanker it has detained in Gibraltar, accusing it of acting at the bidding of the United States.
A senior foreign ministry official "described the UK move as unacceptable" in a meeting with British ambassador Rob Macaire, who had been summoned to hear a formal protest, the ministry said in a statement.
He "called for the immediate release of the oil tanker, given that it has been seized at the request of the US, based on the information currently available," the statement added.
Authorities in Gibraltar, a British overseas territory on Spain's southern tip at the western entrance to the Mediterranean, said they suspected the tanker was carrying crude to Syria in violation of EU sanctions.
The detention of the 330-metre (1,000-feet) Grace 1 vessel comes at a sensitive time in Iran-EU ties as the bloc mulls how to respond to Tehran announcing it is poised to breach the uranium enrichment limit it agreed to in a troubled 2015 nuclear deal.
The Grace 1 tanker was halted in the early hours of Thursday by police and customs agencies in Gibraltar, aided by a detachment of British Royal Marines.
The ship was detained 2.5 miles (four kilometres) south of Gibraltar in what it considers British waters, although Spain, which lays claim to the territory, says they are Spanish.
It was boarded when it slowed down in a designated area used by shipping agencies to ferry goods to vessels.
"We have reason to believe that the Grace 1 was carrying its shipment of crude oil to the Banyas refinery in Syria," Gibraltar's Chief Minister Fabian Picardo said in a statement.
"That refinery is the property of an entity that is subject to European Union sanctions against Syria.
"We have detained the vessel and its cargo," Picardo said.
At US request
Spanish Foreign Minister Josep Borrell told reporters the vessel was detained at the request of the United States.
In a statement, Britain's Foreign Office said: "We welcome this firm action by the Gibraltarian authorities, acting to enforce the EU Syria Sanctions regime."
European Union sanctions against war-torn Syria have been in force since late 2011.
The 28-member bloc has imposed sanctions on Syrian officials including government ministers over their role in the "violent repression" of civilians.
It has frozen the assets of around 70 entities and introduced an embargo on Syrian oil, investment restrictions and a freeze on Syrian central bank assets within the EU.
The tanker's detention comes just days after Iran announced it would exceed the uranium enrichment limit set up as part of the 2015 deal to avoid it building up to the level required for a nuclear warhead.
Tehran took the action in response to Washington abandoning the nuclear deal last year and hitting Iran's crucial oil exports and financial transactions with biting sanctions.
The unilateral move has sent tensions in the Gulf soaring as the administration of President Donald Trump forges ahead with a policy of "maximum pressure" against Iran in coordination with its Middle East allies Israel and Saudi Arabia.
White House National Security Advisor John Bolton, a champion of the hawkish policy towards Tehran, applauded the interception of the supertanker.
"Excellent news: UK has detained the supertanker Grace I laden with Iranian oil bound for Syria in violation of EU sanctions," Bolton tweeted.
According to specialised shipping trade publication Lloyd's List, which analyses vessel-tracking data, the 1997-built ship is laden with Iranian oil.
It reported that the ship loaded oil off Iran in April and sailed around the Cape of Good Hope in South Africa.