Gibraltar authorities said they continued to investigate an Iranian tanker that was boarded and seized by British Royal Marines on suspicion of smuggling Iranian oil to Syria, in violation of European Union sanctions.
U.S. officials hailed the seizure, which occurred July 4, while Iran's Foreign Ministry demanded the ship be released and a former top commander with Iran's Revolutionary Guard called for Iranian authorites to seize British oil tankers in retaliation.
A spokesman for the government of Gibraltar, which is a British protectorate, said July 5 that authorities were continuing to investigate the ship, and all 28 crew members remained on the vessel.
The crew was made of mainly Indian, Pakistani, and Ukrainian nationals, he said, and no arrests had been made yet.
Iran, meanwhile, summoned the British ambassador in Tehran over what it called the "illegal seizure" of the tanker.
"After the illegal interception of an Iranian oil tanker in the strait of Gibraltar by the English naval force, that country's ambassador to Tehran was summoned to the Foreign Ministry," ministry spokesman Abbas Moussavi said.
And Mohsen Rezaee, secretary of Iran’s influential Expediency Council, said Iran should seize a British vessel if the tanker is not released. Rezaee is a former leader of Iran's powerful Islamic Revolutionary Guards Corps.
“If Britain doesn’t return the Iranian tanker, the duty of responsible [Iranian] bodies is to seize a British oil tanker in a retaliatory measure,” Rezaee tweeted on July 5.
The ship, identified as the Grace 1, was detained 4 kilometers south of Gibraltar in what Britain considers its territorial waters, although Spain -- which still claims the territory -- says they are Spanish waters.
"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.
The European Union has banned oil shipments to Syria since 2011, but this would be the first time a tanker has been seized at sea.
The United States said it welcomed the seizure of the tanker. White House national-security adviser John Bolton said in a post to Twitter that it was "excellent news."
"America & our allies will continue to prevent regimes in Tehran & Damascus from profiting off this illicit trade," Bolton wrote.
Matthew Oresman, a lawyer who specializes in sanctions, said the seizure was the first time the EU had done something so aggressive and public.
The seizure comes at a time of high tensions between the West, especially the United States and Iran.
Last year, U.S. President Donald Trump withdrew from the 2015 nuclear deal that curtailed Iran's nuclear programs, and began reimposing crippling sanctions against Tehran.
Trump has also increased U.S. troop numbers in the Middle East, and ordered a U.S. aircraft carrier battle group to the region.