British marines reportedly seized an Iranian oil tanker Thursday for attempting to transport oil to Syria in violation of European Union sanctions.

The tanker was impounded in Gibraltar on the southern tip of Spain after sailing around Africa, according to Reuters.

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Though the ship flies a Panama flag and is listed as managed by a Singaporean company, Iran’s Foreign Ministry summoned the British ambassador to declare “its very strong objection to the illegal and unacceptable seizure.”

The Gibraltar government said it had good reason to believe the tanker was carrying crude oil to the Baniyas refinery in Syria.

“That refinery is the property of an entity that is subject to European Union sanctions against Syria,” Gibraltar Chief Minister Fabian Picardo told Reuters. “With my consent, our port and law enforcement agencies sought the assistance of the Royal Marines in carrying out this operation.”

Europe has banned oil shipments to Syria since the start of its civil war in 2011 but has never before seized a tanker.

“This is the first time that the EU has done something so public and so aggressive. I imagine it was also coordinated in some manner with the U.S. given that NATO member forces have been involved,” Matthew Oresman, a partner with law firm Pillsbury Winthrop Shaw Pittman who advises firms on sanctions, told Reuters.

“This is likely to have been meant as a signal to Syria and Iran — as well as the U.S. — that Europe takes sanctions enforcement seriously and that the EU can also respond to Iranian brinkmanship related to ongoing nuclear negotiations.”

The EU’s sanctions on sending oil to Iran are separate from stringent U.S. sanctions imposed against Iran’s oil industry and top Iranian officials.

The news comes as the EU tries to work with Tehran to keep it within the bounds of the Obama-era nuclear pact from which President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE withdrew the U.S. last year.

Tehran recently announced that it would continue enriching uranium beyond the limitations of the agreement amid skyrocketing tensions with Washington, alarming several European signatories of the deal.

Trump warned Iran about making threats over its uranium enrichment in a Thursday morning tweet.