Iran demanded Friday that Britain immediately release an oil tanker it has detained in Gibraltar, accusing it of acting at the bidding of the US.

A senior foreign ministry official "described the UK move as unacceptable" in a meeting with British Ambassador to Iran Rob Macaire, who had been summoned to hear a formal protest, the ministry said in a statement. 

The official "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."

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

The detention of the 330-meter Grace 1 vessel 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 tanker was halted in the early hours of Thursday by police and customs agencies in Gibraltar, aided by a detachment of British Royal Marines.

It 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.

"That refinery is the property of an entity that is subject to European Union sanctions against Syria.

"We have detained the vessel and its cargo," Picardo said.

Spanish Foreign Minister Josep Borrell told reporters the vessel was detained at the request of the US.

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".

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.