Iran’s President Hassan Rouhani yesterday warned Britain of “consequences” over the detention of an Iranian oil tanker off the coast of Gibraltar.

“I point out to the British that you initiated insecurity (on the seas) and you shall grasp the consequences of it later on,” Rouhani said in comments to the cabinet broadcast by state TV.

Iran’s president called the seizure of the tanker “a foolish act.”

The 330 metre (1,000 feet) Grace 1 tanker, capable of carrying two million barrels of oil, was halted last Thursday by police and customs in Gibraltar -- a tiny British overseas territory on Spain’s southern tip -- with the aid of a detachment of British Royal Marines.

Iran condemned the detention as an “illegal interception,” but Gibraltar officials said that the cargo was believed to be destined for Syria, which is subject to European sanctions.

Iran denied this, saying that the destination “was somewhere else.”

“The port named in Syria does not even have the capacity for such a supertanker to dock,” Iran’s Deputy Foreign Minister Abbas Araghchi said Sunday.

Iran’s defence minister vowed Monday to respond to Britain’s move.

Meanwhile, President Donald Trump yesterday warned that sanctions against Iran would soon be “increased substantially” after Tehran said it had exceeded a limit on enriched uranium reserves under a 2015 nuclear deal abandoned by Washington.

“Iran has long been secretly ‘enriching,’ in total violation of the terrible 150 Billion Dollar deal made by John Kerry and the Obama Administration,” Trump said on Twitter.

“Remember, that deal was to expire in a short number of years. Sanctions will soon be increased, substantially!”

European powers urged Iran on Tuesday to reverse Iran’s move to increase uranium enrichment, as a French envoy arrived in Tehran to boost efforts to save the nuclear deal.

“It must act accordingly by reversing these activities and returning to full JCPoA compliance without delay,” said a statement from the European Union and foreign ministers of France, Germany and Britain.

Amid fraught relations between Washington and Tehran, General Joseph Dunford, chairman of the Joint Chiefs of Staff, told media on Tuesday that the US aims to form a coalition to guarantee freedom of navigation in strategic Gulf waters.