CNN  — 

British Royal Marines in Gibraltar stormed an Iranian ship believed to have been carrying oil to Syria on Thursday, in possible violation of European Union sanctions on Syria.

Gibraltar port and law enforcement agencies, aided by the British marines, detained the Grace 1 super tanker early Thursday morning, the territory’s chief minister, Fabian Picardo, said in a statement.

Grace 1 was boarded by Royal British Marines and Gibraltarian authorities.

Spain’s acting foreign minister, Josep Borrell, said the United States had asked the UK to intercept the ship.

“We have reason to believe that the Grace 1 was carrying its shipment of crude oil to the Baniyas Refinery in Syria,” Picardo said. “That refinery is the property of an entity that is subject to European Union sanctions against Syria.”

The EU imposed a number of financial, trade and transport sanctions against Syrian President Bashar al-Assad’s regime in 2011 in response to “the continuing brutal campaign” against its own people.

The super tanker is currently anchored off the coast of Gibraltar.

Iran demands the tanker’s release

A spokesman for Iran’s Foreign Ministry, Abbas Mousavi, tweeted that the British ambassador to Tehran had been summoned over the “illegal” seizure of the oil tanker.

A senior Iranian Foreign Ministry official condemned the operation by the Royal Marines, calling it “piracy,” according to a report from the semi-official Tasnim news agency on Friday.

The official said the UK does not have the right to implement its or the EU’s unilateral sanctions against other nations “in an extraterritorial manner” and called for the vessel to be released immediately after it had been seized “at the behest of the US.”

The vessel was suspected of carrying oil to Syria, breaching European Union sanctions.

Mousavi said Thursday that the “destructive” seizure of the oil tanker by the UK could increase tensions in the Persian Gulf, the state-run Press TV reported, citing a statement from Mousavi.

Iranian state news agency IRNA said Royal Marines boarded the 300,000-ton carrying capacity ship early Thursday morning.

The incident comes at a time of heightened tensions between the US and Iran. Earlier this week, Iran announced it would stop complying with several parts of the 2015 nuclear deal that the Trump administration withdrew from last year.

The Syrian government has not yet responded to the incident.

Grace 1 is anchored off the coast of Gibraltar, a British overseas territory on the edge of southern Spain.

The UK’s Ministry of Defense (MOD) welcomed what it called “firm action by the Gibraltarian authorities, acting to enforce the EU Syria sanctions regime.”