The tanker's seizure "will not be tolerated by us and will not go without a response", said Amir Hatami, quoted by Iran's ISNA and Tasnim news agencies.

"This move is against international regulations and a kind of maritime piracy," he said during a ceremony at Bandar Abbas port in southern Iran.

The 330-metre (1,000-feet) Grace 1, capable of carrying two million barrels of oil, was halted Thursday by police and customs agencies in Gibraltar, a British overseas territory on Spain's southern tip.

Authorities in Gibraltar said they suspected the tanker was carrying crude to Syria in violation of EU sanctions.

Tehran denies this and claims the vessel was intercepted in international waters.

The tanker's detention "sets a dangerous precedent and must end now", Iran's Foreign Minister Mohammad Javad Zarif tweeted Monday.

He questioned the seizure's legality, saying "Iran is neither a member of the EU nor subject to any European oil embargo".

Europe is supposed to be "against extraterritoriality" unlike the United States, he wrote.

"UK's unlawful seizure of a tanker with Iranian oil on behalf of #B_Team is piracy, pure and simple," Zarif said.

He regularly uses the term "B  Team" to refer to US National Security Advisor John Bolton, Israeli Prime Minister Benjamin Netanyahu, and the Saudi and Abu Dhabi crown princes, Mohammed bin Salman and Mohammed bin Zayed, who are all pushing a hard line on Tehran.

Gibraltar's Supreme Court ruled Friday that the tanker can be held for 14 more days, the territory's attorney general said.

The government of Gibraltar said in a statement Monday that the tanker was "well inside" British Gibraltar Territorial Waters when it was boarded, having "previously exited the international waters of the Straits of Gibraltar".

It released a screenshot along with the statement of a maritime map that showed the tanker's location.

The vessel's detention comes at a sensitive time in Iran-EU ties as the bloc mulls how to respond to Tehran breaching the uranium enrichment limit it agreed to in the troubled 2015 nuclear deal.