Iran demanded 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.
The secretary of Iran's Expediency Council, a key advisory and arbitration body, warned that if Britain failed to release the tanker Iran would be forced to take tit-for-tat action against a British vessel.
"In its 40-year history, the Islamic revolution has never been the source of any tension but in the face of arrogance, we have never hesitated to respond," said council secretary Mohsen Rezai.
"If Britain does not release the Iranian oil tanker, the relevant authorities will be duty-bound to take reciprocal action and seize a British oil tanker."
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 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 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.
- 'Maritime banditry' -
Iran insists the vessel was intercepted in international waters and the foreign ministry official accused Britain's Royal Navy of taking action "tantamount to maritime banditry".
"Britain has no right to impose its own unilateral sanctions or those of the European Union in an extraterritorial manner against the other countries," the official said.
The Gibraltar authorities say the tanker 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, Britain's Foreign Office said: "We welcome this firm action by the Gibraltarian authorities, acting to enforce the EU Syria sanctions regime".
Spanish Foreign Minister Josep Borrell told reporters the vessel was detained at the request of the United States.
But Gibraltar insisted it acted of its own accord.
"There has been no political request at any time from any government," Gibraltar's government said in a statement.
"The decisions of Her Majesty's Government of Gibraltar were taken totally independently, based on breaches of existing law and not at all based on extraneous political considerations."
- 'Excellent news' -
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.
The tanker's detention comes just days after Iran announced it would exceed the uranium enrichment limit set under the 2015 deal to allay concerns it might seek to develop a nuclear weapons capability, something it has always denied.
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.